I am kind of confused on this problem.

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

Solve for t_air in the air using h = 1/2 g t_air² as if the projectile was just dropped to the ground.

use time in the air to solve for the x position using x = vt_air - the x velocity is unaffected by gravitational acceleration. x and y components of velocity are independent.