The first thing you should know is that you have a system of two equations with two unknowns.
Clearing we have:
y-x = 9
2y + 2x = -10
Step1: multiply equation 1 by -2
-2y + 2x = -18
2y + 2x = -10
Step2: add both equations:
4x = -28
x = -28 / 4 = -7
answer
The x-coordinate of the solution for the system of equations is x = -7
