The quantity demanded for a certain brand of MP3 players is 200 units when the price is set at $90. The quantity demanded is 1,200 units when the unit is $40. Find the demand equation.

For this problem, I set the points as (demand quantity, price). So my points were (200,90) and (1200,40). I then proceeded to look for slope m.
m= 40-90/1200-200 = -50/1000 =-1/20

Then used point-slope (I plugged in the points from (200,90)):
y-90= -1/20 (x-200)
y-90+90= -1/20x+10+90
y=-1/2-x+100 as my final answer.

I need someone to check my work.

I think you have thongs backwards. Since you can set the price at will, it should be the independent variable. So, your points should be

(90,200) and (40,1200)
That makes the slope of the line (1200-200)/(90-40) = 20
So, the equation is
y-200 = 20(x-90)
y - 20x - 1600

Other than that, you did good work :-)

Your work is almost correct, but there is a small mistake in the calculation of the slope.

To calculate the slope, you need to subtract the y-coordinates and divide by the difference in x-coordinates. So, it should be:

m = (40 - 90) / (1200 - 200) = -50 / 1000 = -1 / 20

After getting the correct slope, you can use the point-slope form as you did before to derive the demand equation. Using the point (200, 90):

y - 90 = (-1/20)(x - 200)

Now, let's simplify it further:

y - 90 = (-1/20)x + 10

Add 90 to both sides:

y = (-1/20)x + 100

So, the demand equation is y = (-1/20)x + 100, where y represents the quantity demanded and x represents the price.