Exercise 1 solution

The echo travels down and back, so the one-way time is 6.0 / 2 = 3.0 s.
Depth = 1,500 m/s x 3.0 s = 4,500 m.

At about 4,500 m the likely feature is an abyssal plain (or the continental rise at its upper edge). It is too deep for a shelf or slope and not deep enough for a trench.