A spring with a force constant of 225 N/m is resting on a frictionless surface and mounted horizontally on a wall. A 1.5 kg box is pushed against the spring, compressing it 12.0 cm from equilibrium. When released, the spring pushes the box across the floor. a) How much force must be exerted on the spring to compress it 12.0 cm? b) How much elastic potential energy is stored in the spring while it is compressed? c) Once released, what maximum speed will the box attain?