For the following system, if you isolated x in the first equation to use the substitution method, what expression would you substitute into the equation? -x-2y=-4 3x+y=12

Do you mean substitute into the second equation?
If that is so
-x = 2y - 4
x = -2y + 4 You could put this into the second equation. When you do
3(-2y + 4) + y = 12
-6y + 12 + y = 12
-5x =0
x = 0.

y = 12