solve the system of equations using substitution.

y = x + 10
y = 3x

a. (5, 15)
b. (2.5, 7.5)
c. (–15, –5)
d. (–5, –15)

3x = x + 10

Solve for x, then put that value in the second equation (above) to find y. Put both in the first equation to check.

To solve the system of equations using substitution, we need to substitute the value of one variable into the other equation and solve for the remaining variable. Let's solve the system of equations step by step.

First, we have the two equations:
1) y = x + 10
2) y = 3x

To use substitution, we'll substitute the expression for 'y' from equation 1) into equation 2). This means we'll substitute 'x + 10' for 'y' in equation 2):

x + 10 = 3x

Now, we'll solve this equation for 'x'. Subtract 'x' from both sides of the equation:

10 = 2x

Divide both sides of the equation by 2:

5 = x

Now that we have found the value of 'x', we can substitute it back into either equation to find the value of 'y'. Let's substitute 'x = 5' into equation 1):

y = 5 + 10
y = 15

So, the solution to the system of equations is (x, y) = (5, 15).

Therefore, the correct answer is a. (5, 15).