f(x) = (x-4)(x+1)^(2/3)f'(x) = (x+1)^(2/3) + (2/3)(x-4)(x+1)^(-1/3) = [3(x+1)+2(x-4)]/[3(x+1)^1/3)] = 5(x-1)/[3(x+1)^1/3)]
