If you want the x-coordinate of the solution of the system of lineare equations
y - x = 9
10 + 2x = -2y
then you must eliminate y from this sytem. Rewrite the first equation: y - x = 9 becomes y = x + 9. Substitute this (x + 9) for y in 10 + 2x = -2y. y will now be gone. Solve the resulting equation for x and you'll be done.