x-y = 10 (eq. 1)
y = 2x-7 (eq. 2)
Substituting the value of y from the second equation into the first equation, we get:
x - (2x-7) = 10
x - 2x + 7 = 10 removing parentheses
-x + 7 = 10 combining similar terms
-x + 7 - 7= 10 - 7 subtracting 7 at both sides
-x = 3
-x/(-1) = 3/(-1) dividing by -1 at both sides
x = -3
Replacing this result into the second equation,
y = 2(-3) - 7
y = -6 - 7
y = -13