Flight of a rocket the altitude in feet of a rocket t sec

Question

Question: Flight of a rocket the altitude (in feet) of a rocket t sec into flight is given by

s = f(t) = -t³ + 54t² + 480t + 6             (t ≥ 0)

Find the point of inflection of the function f and interpret your result. What is the maximum velocity attained by the rocket?