The Potential Energy for a Certain Mass Moving in One

The potential energy for a certain mass moving in one dimension is given by U(x) = (2.0 J/m³) x³ - (15 J/m²) x² + (36 J/m) x - 23 J. Find the location(s) where the force on the mass is zero.

A) 4.0 m, 5.0 m
B) (3.25 - 0.968i) m, (3.25 + 0.968i) m
C) 1.0 m
D) 2.0 m, 3.0 m
E) 3.0 m, 5.0 m