If a rock is dropped off of a sea cliff, and the sound of the rock hitting the water is heard 3.4 seconds later, how tall is the cliff, assuming the speed of sound is 340 m/s. What I've been trying to do is break the problem up into 2 parts, one for the rock going down towards the sea, and one for the speed of sound going up the cliff. The problem is that I can't seem to substitute the right things into the equation x = x0 + v0t + .5at^2. For the going down part, the equation looks like the following: X= 0 + 0 + (.5)(9.8)t^2. And for the going up part, I'm not sure whether the initial velocity of the speed of sound is 340 m/s, or whether it is zero. If the initial speed were 340 m/s, then the equation would (?) look like the following: x = 0 + 340t + 0. This is where I get stuck, and I don't know what to do with the two equations. I would greatly appreciate the help. Thanks.