Barrons AP Calculus - David Bock

12. (E) f ′(x) = 4x^3 − 8x = 4x(x^2 − 2). f ′ = 0 if x = 0 or

f ′′(x) = 12x^2 − 8; f ′′ is positive if x = negative if x = 0.