My son has been given the collinear problem below that has be stumped two folds.

1- By trying to solve the problem using (y2-y1)/(x2-x1)=slope -- and then, find the (b) in y=mx+b for two points takes way way too long to be a 7th grade probem. My son has already been working on it for over two hours. There must be a shortcut…
2- Why is it that; y=mx+b for points with a zero never give the righ b? e.g. (0,3)
Problem:
The coordinates of the points are:
A= (0,3) | B=(2.11) | C=(3,-3) | D=(-1,9) | E=(-3,10) |F=(6,10) | G=(-5,19) | H=(-6,18)
Three, and only three of these points are collinear. The student’s task is to determine “Analytically” and without plotting, which three is the collinear trio.

sorry B=(-2,11)

I agree with you that this is a rather ridiculous question

There are 21 possible pairs of points, each pair could give us the equation of a line containing such a pair.

If the objective is to use the method you suggest, it could just as easily be effictive with , let's say, 4 points.

I would have allowed my students to make a sketch, thus eliminating some of the more obvious non-collinear points.

You don't actually have to find the equation of the line. If you find the slopes of pairs that have a common point, and those two slopes are equal, then the 3 points are collinear.
e.g. If slope PQ = slope PR , the P, Q, and R are collinear, notice P was the "link"
but, if slope PQ = slope RS, all we know is that PQ || RS

As to your point #2, I think you must be making some kind of error,
Using a point such as (0,3) in y=mx +b will let you find b in the same way as using any other point

e.g. suppose we know y = 4x + b and we are told that (0,3) is on it
3 = 4(0) + b
b = 3
notice that in y = mx + b, the b value is the y-inercept
and of course (0,3) is the point of the y-intercept, so obviously if in a point, the x = 0, the y must be the value of b in the equation.

I played with this for a while but could see no practical way to do it without sketching the points.

I dis put the points in order of increasing x from -6 to +6 which makes it easier to visualize

Thank you very much Reiny and also Damon. This is the first time I felt embarrassed helping my son. Sign of things to come of course. As Reiny suggested, we ended up plotting and then selecting less than half a dozen candidates. I also cheated and made a spreadsheet just to speed things up. The spreadsheet quickly showed the Trio is: C, H AND F.

Thank you again

now you guys are 20 years old

To solve the collinear problem for your son, there are a few different approaches you can take. Let me explain each method in a step-by-step manner:

Method 1: Using the slope-intercept form (y = mx + b):
1. Choose any two points from the given coordinates. Let's say we pick points A and B, with A = (0, 3) and B = (2, 11).
2. Calculate the slope (m) using the formula: m = (y2 - y1) / (x2 - x1), where (x1, y1) and (x2, y2) are the coordinates of the two points. In this case, m = (11 - 3) / (2 - 0) = 8 / 2 = 4.
3. Now that you have the slope, substitute it into the equation y = mx + b, along with the coordinates of one of the points (let's use A). The equation becomes 3 = 4(0) + b, which simplifies to 3 = b.
4. So the equation for the line passing through A and B is y = 4x + 3.
5. Repeat steps 2-4 for each pair of points. If the resulting equation is the same for three points, those points are collinear.

Method 2: Using determinants:
1. Represent each point as a matrix, where the first row contains the x-coordinate and the second row contains the y-coordinate. For example, A can be represented as A = [[0],[3]].
2. Create a 3x3 matrix using the points you want to check for collinearity. For instance, for points A, B, and C, the matrix would be [[0, 2, 3], [3, 11, -3]].
3. Calculate the determinant of this 3x3 matrix. If the determinant is zero, the points are collinear.

Now let's address the second part of your question:

Points with a zero x-coordinate will not give the right value for b in the equation y = mx + b because the x-variable is multiplied by the slope (m). When the x-coordinate is zero, the product of m and x becomes zero, so any term involving x disappears. Therefore, the equation reduces to y = b, where b is the y-intercept. In this case, the y-intercept is directly equal to the value of y when x = 0. So, for example, in the point (0, 3), the value of b would be 3.

I hope this explanation helps your son solve the collinear problem more efficiently and understand the concept better.