Physics-please help?

Since perfectly elastic collisions ume an instantaneous change in velocity ( in this case, a reversal), the ball travels 12m from one wall, reflects and travels 12 m back to its original position, where it reflects again. With no energy loss, this goes on interminably. The period is simply 24 m / 3 m/s = 8 seconds. If you graph the position of the ball versus time, you see a triangular waveform. Simple harmonic motion has a sinusoidal waveform.