physics problem!

here's a really hard physics problem for ya! I solved it, maybe you can!

here it is:

A rock is dropped off a cliff into the ocean. The sound of the splash is heard 3.2 seconds after it is dropped. Assuming the speed of sound is 340 m/s and gravity accelerates the rock at 10 m/s^2, what is the height of the cliff? (neglecting air resistance)
help:

Find givens:
Time: 3.2sec
Sound spd.: 340m/s
Gravity: 20m/s^2

Unknowns:
Time of sound: ?
Time of fall: ?
height: ?

DRAW A DIAGRAM!

Eqn's: You'll need 2 of them! Find out which ones!(all require assumed constant acceleration)
x=xo+vot+(.5)g(t^2)

v=v+at

(avg.)v=(vo+vf)/2

(avg.)v=d/t

GOOD LUCK! HAVE FUN!