Answer:
2625 m deep
Step-by-step explanation:
Let the sound speed in sea water be 1500 m/s. If he hears the echo 3.5s after the strike, then the sound would have traveled a distance of 1500 * 3.5 = 5250 m to the bottom and back. This would mean the ocean is 5250 / 2 = 2625 m deep.