Projectile motion- Deriving the right formular

[majormuss]

Homework Statement

Projectile A is launched horizontally at a
speed of 20. meters per second from the top of
a cliff and strikes a level surface below, 3.0 seconds
later. Projectile B is launched horizontally
from the same location at a speed of 30. meters
per second.
Approximately how high is the cliff?
(1) 29 m (3) 60. m
(2) 44 m (4) 104 m

Homework Equations

d=tv

The Attempt at a Solution

On two occasions my answer turn out to be '3' and '4'. but the answer key says it's '2'. I have tried for hours but i can't find the right approach. Please offer me a good explanation of how I should work with this? or u can even give me a link to read more or something.

 

[rl.bhat]

Use the equation
h = 1/2*g*t^2