get y in terms of x then substitute the expression into the other equation.
#7.
y - 2x = -6
y = 2x -6
y gets substituted for 2x - 6
5x - (2x - 6) = 9
5x - 2x + 6 = 9
3x + 6 = 9
3x = 9 - 6
3x = 3
x = 1
use this to find the other one
y - 2(1) = -6
y - 2 = -6
y = 2 - 6
y = -4
#8.
3y + x = 2
x = 2 - 3y
substitute x for 2 - 3y in the second equation
y - (2 - 3y) = -6
y - 2 + 3y = -6
4y - 2 = -6
4y = 2 - 6
4y = -4
y = -1
use this to find the other one
-1 - x = -6
-x = 1 - 6
-x = -5
x = 5