You are given V(x) = 4x - x^2 = x*(4-x). The roots of this parabola are x= 0 and x= 4 -- hence, the maximum of the parabola is half way between the roots, i.e. at x= 2. It means that the maximum speed is (substitute x= 2 into the given formula)= 4*2 - 2^2 = 8 - 4 = 4 m/s. ANSWER