To solve this question you can just sub one equation in to the other.

In this case:

3x - y = -15 (equation 1)

The second equation shows that x = y - 7 (equation 2)

CORRECT SOLUTION

You can replace the x in the first equation, with y - 7, since in the second equation shows that x is the same as y - 7

3 (y - 7) - y = -15

Multiply the 3 into the y - 7

3y - 21 - y = -15

Add 21 to both sides

2y - 21 + 21 = -15 + 21

2y = 6

Divide 2 from both sides

2y/2 = 6/2

y = 3

To get x simply subsitute the y back into the original equations

In this case I will subsitute into equation 2

x = y - 7 , since y =3

x = 3 - 7

x = -4