P'(x) = x^3 - x^2 - 4x + 4
= x^2(x - 1) - 4(x - 1)
= (x^2 - 4)(x - 1)
= (x+2)(x-2)(x-1)
so P'(x) = 0 for x =2,1,-2
P(1) = 23/12 + c
P(2) = 4/3 + c
P(-2) = c - 28/3
so P(x) > 0 for all stationary points (hence all x since P increases without bounds for large and small...