You need to use newton's third law of motion...
max height height rocket ascends
acceleration due to gravity (32.1522 ft/s it is negative as it is accting downward)
initial vel(900 ft/s)
final vel(0 m/s at top pos it doesnt have velocity)
Where time in the air.
This is, in my opinion, a really bad problem. The equation Anonymous1 gives, as well as will work for something thrown upward so that the only force is that of gravity. But the whole point of a rocket is that it continues firing as it goes up so that is NOT true.