1. The problem statement, all variables and given/known data
A ship sends a sonar signal to the bottom of the ocean, 4000 m below. The speed of sound in the seawater under the conditions of the problem is 1500 m/s How much time elapses between the transmission of the signal and the reception of its echo?

You have [itex]v_s/d = 1/t[/itex] and if you want to multiply both sides by d, you will also want to multiply both sides by t (you did something weird). Your first eqn. is okay, but the second one is wrong. So you will want to find the time it takes to go to the bottom, and then back up (they happen to be equal after a moments thought) to find the total time.