Physics problem on mechanics

I have a following problem

A rock is dropped from a sea cliff. After 3.4 s a splash of the rock hitting the ocean is heard. The speed of sound is 340 m/s. How tall is the cliff?

I came to the conclusion that the lenght of the cliff equals the lenght of the distance the sound travels, and that the time the rock travel and the time the sound travels should equal 3.4 seconds, but I don't know whether this is correct and how to write this in symbols.

Can someone please help me?

Thank you!