I am stuck on this question, here it is.

A ball with mass m kg is thrown upward with initial velocity 28m/sec from the roof of a building 11m high. Neglect air resistance.

(a)Find the maximum height above the ground that the ball reaches.

(b)Assuming that the ball misses the building on the way down find the time that it hits the ground.

I found the answer to part (a) as 51 meters but I can't get part (b). Thanks for any help!