Hint:

Draw a diagram and let d₁ be the distance to the closer cliff and let d₂be the distance to the further cliff. The time to hear the first echo is the time it takes the sound to travel to the closer cliff and back,

. The time to hear the second echo is the time it takes sound to travel to the further cliff and back,

. The third echo is heard when the sound makes a complete round trip (to one cliff, to the other, and back to the person) so that

. Use the fact that

and

to set up simultaneous equations for d₁ and d₂.