you have to isolate the x or the y. So move the 5x to the other side by subtraction,-4y=8-5x, then divide each part by -4, -4y/-4=(8/-4)-(5x/-4). Gives you y=-2+5/4x.
Now plug that y into the original equation. 5x-4(-2+5/4x)=8...
then once you get x plug that into the original equation and solve for y