-x - 7y = 18
4x + 7y = -30
The -7y and 7y cancel by addition.
Thus, 3x = -12
Divide both sides by 3
x = -4
Plug in -4 for x
Thus, -(-4) - 7y = 18
4 - 7y = 18
Subtract 4 to both sides; 4 - 4 - 7y = 18 - 4
Simplify, -7y = 14
Divide both sides by -7; -7y/-7 = 14 / -7
y = -2
