I am confused on how to do this physics question

Question

A ball projected horizontally with an initial velocity of 20 m/s east, off a cliff 100 meters high. It takes 4.5 s for the ball to reach the ground. [Neglect air resistance] 

How far from the base of the cliff does the ball land?

Answer 1

Considering downward direction

s = ut + 0.5 at^2

s = 100, u = 0, a = 9.81, giving t = 4,52 s

So ball hits ground after 4.52s

Horizontal speed is constant (no net force in horizontal direction) so horizontal distance after 4.52s

= 20 x 4.52 = 90.4 m

This is your answer