21. To stretch a certain nonlinear spring by an amount x requires a force F given by F=40x−6x2, where F is in newtons and x is in meters. What is the change in potential energy when the spring is stretched 2 meters from its equilibrium position?
(A) 16 J
(B) 28 J
(C) 56 J
(D) 64 J
(E) 80 J
(A) 16 J
(B) 28 J
(C) 56 J
(D) 64 J
(E) 80 J