How would I go about answering this question?

A particle of mass m moves freely in the interval [0,a] on the x-axis (so that the potential V = 0 within the interval). Initially the wave function is

.

Show that at a later time t the wave function is

.

Note that the h's in the formula above are supposed to be h-bars, but I don't know how to write them in LaTeX.

Thanks to anyone for any help you can give me!