Answer:
U = 17.15m/s.
Explanation:
The ball rises up a height h. After reaching this height the ball falls back down this same height. The time taken to rise the height h is also equal to the time taken to fall the same height.
So the total time taken for the flight 3.50s is equal to 2× the time taken to rise to height h, t.
See the attachment below for the complete solution.