Horizontal displacement =v0t [as the acceleration is zero]
In a horizontal projectile, horizontal velocity remains constant.
it is given that at a certain time horizontal velocity is equal to vertical velocity
vy = v0
Generally, vy = u + gt [u = 0]
v0 = gt
t = v0 /g ----------(1)
As given above horizontal displacement at time t = v0t ----------(2)
substitute the above equation (1) in (2)
sx = v0 (v0 /g)
sx = v0 2 /g