At some time t=T, consider a snapshot of the graph. Focus on particular point (a bump, if wish) located at x=X on the graph. So, that feature has height H=g( 0.5X - 3T ). As T increases, what happens to that feature [keeping that height the same (which means keeping constant (0.5X - 3T) ]?
It might help to redraw your function at various times t (as suggested in your assignment).