In each of the problems, one of the equations is solved for y. Just plug in what y is equal to into the other equation.
1.) Plug -3 in for y.
-3 = 7x - 10
7x = 7
x = 1
2.) Plug -8 in for y.
-8 = -2x - 12
-2x = 4
x = -2
9.) Plug -8x in for y.
2x + 4(-8x) = 0
2x - 32x = 0
-30x = 0
x = 0
10.) Plug 2x in for y.
6x + 7(2x) = 20
6x + 14x = 20
20x = 20
x = 1