All you have to do is substitute the y value into the second equation.
8x - (-4x) = 6; double negative turns it to a positive
8x + 4x = 6; combine like terms
12x = 6; divide both sides by 12
x = 1/2 then substitute this x value into the second equation again
8(1/2) - y = 6; multiply
4 - y = 6; subtract 4 from both sides
-y = 2; multiply both sides by a negative to make y positive again
y = -2
The solution is (1/2, -2)