x = 4, y = 6
In this case, you can solve the equation by substitution.
3x + 4y = 36
We are given that y = -1/2x + 8
Plug in that value assigned to y.
3x + 4(-1/2x + 8) = 36
3x -2x + 32 = 36
x + 32 = 36
x = 4
Now, we can return back to y
y = -1/2x + 8
y = -1/2(4) + 8
y = -2 + 8
y = 6