Now "time until bottom" means the amount of time the rock hits the water. But when it hits the water it takes time for the sound to travel up. That is what I mean by "time until top". In total, this trip takes 6.1 seconds.
Let be the length of the well.
That means "time until bottom" is . (Why is that. This is an important part of this solution).
Now "time until top" is simply the distance formula. Thus, .
Thus, we have an equation,
Now solve it.