What is the x-coordinate of the solution for the system of equations? {y−x=910+2x=−2y?

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

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