You're doing it correctly, including the classification. You just need to keep going. For A, where you stopped for some reason, plug f and df/dx in and you're done. (Actually, you don't even need to do that much work, since you're evaluating it at 3 points (0,.5), (0,-.5), (.25,0) where you already know df/dx=0. Thus A(P)=4f(P) when P is one of those 3 points.)