Projectile motion rock off a cliff

[maddief]

Homework Statement

A rock is thrown down off a cliff at 18 m/s at an angle of 17 below the horizontal. If the rock hits a cliff that is 16m away, how far down did the rock fall.

Homework Equations

Vf= Vi + at
change in Y= Vit + .5at^2

The Attempt at a Solution

I tried solving for time and then plugging that back into the equations and I kept getting -3.87. Please help!

 

[CAF123]

You can eliminate time by writing an equation for the x displacement of the rock and subbing this into your equation for it's y displacement.