In order to solve this system by substitution, simply take the value of y in the first equation and use it in place of y in the second. This will allow you to solve for x.
4x + 4y = 20
4x + 4(x + 5) = 20
4x + 4x + 20 = 20
8x + 20 = 20
8x = 0
x = 0
Now that we have the value of x, we can find the value of y.