Question

A ball is thrown up with a speed of 15 m/s how high will it go before it begins to fall

Answer 1

You should give time period to solve this. You can't find height without TIME.

Because of the formula..

speed = Distance / time.

so, distance = speed × time.......

Answer 2

Find Time of Flight,
Intial velocity = 15m/s
Tf= 2u/g
= 2 into 15 by 10
= 3.061
= 3m/s