Question 242780
The question gives the method for solving the question.  The first equation is x-y=4.  The second equation defines y=2x+6.  Substituting the right-hand side of the second equation in place of y in the first equation obtains:  x - (2x+6) = 4.  This becomes x - 2x - 6 = 4, or -x - 6 = 4.  Add 6 to both sides, we have:  -x = 10.  Multiply by -1 to obtain:   x = -10.
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Substituting in the second equation:  y = 2(-10) + 6 = -20 + 6 = -14.
Substituting in the first equation:  x - y = 4, which is -10 - (-14) = -10 + 14 = 4.