7x - 2y = -4 (1)
-x - y = 7 (2)
To use the substitution method we will find y in the second equation
-x - y = 7 .......... add x to both sides
-x + x - y = 7 + x
-y = 7 + x ......... (3)
Substitute (3) in (1)
7x + 2(7 + x) = -4
7x + 2(7) + 2(x) = -4
7x + 14 + 2x = -4
Add the like terms
(7x + 2x) + 14 = - 4
9x + 14 = -4
Subtract 14 from both sides
9x + 14 - 14 = -4 - 14
9x