velocity problem

Resolve the initial velocity in vertical and horizontal directions.
For horizontal direction:
15 = [20.cos(30)].t
where t is the time of travel before hitting the target
hence, t = 15/20.cos(3) s -------------- (1)

For vertical direction:
use equation of motion s = ut + (1/2)at^2
with u = 20.sin(30) m/s, s = h, a = -g(=-10 m/s2)
h = [20.sin(30)].t + (1/2)(-10).t^2
substitute t from (1)
h = [20.sin(30)].[15/20.cos(3)] - 5.[15/20.cos(3)]^2 m