Let me know if I've got the right answer, but it should go something like this.
So we can call the 2 remaining sides x and 8 - x
Area = x (8 - x) = 8x - x^2
We want to differentiate this in terms of x to find the maximum
dA / dx = 8 - 2x
dA/dx = 0 when x = 4
To prove it is a maximum, we...