The height of the cliff is equal to 80 m.
Your object is thrown from a cliff of unknown height. You know that the initial velocity of the object if equal to 15 m/s, and that it takes the object 4 seconds to hit the ground.
Despite the fact that the object's motion has a horizontal component as well, you'll only work with its vertical component.
The verical component of the initial velocity will be equal to zero. The vertical movement of the object will be influenced by the gravitational acceleration,
Plug in the value you ahve for