The Student Room Group
Use v = u + at vertically:

0 = usin(alpha) - gt

=> t = [usin(alpha)]/g (*)


Now, use speed = Distance/time

ucos(alpha) = AB/t

Substitute in from (*) to get:

ucos(alpha) = [ABg]/(2usin(alpha)

=> AB = 2u^2sin(alpha)cos(alpha)/g

then use trig identity 2cos(alpha)sin(alpha) = sin(2alpha) to get the answer