In the given question the equation has more than two variables and they are “x” and “y”. These kind of equations cannot be properly solved as there are two variables and one equation. For proper solving and finding the value of y, you need to have two equations.
-8x + 4y =20
4y = 20 + 8x
y = (20/5) + (8x/4)
= 4 + 2x
So the value of y as can be determined from this given equation is 4 + 2x