The speed of sound is 343 meters per second. This means the foghorn sound will cover 343 meters in one second. There are two ways you may solve this problem. It took two and a half seconds to make the whole trip, for the foghorn sound to start, reflect, and make it back to the tugboat. 343 meters times 2.5 seconds gives a total of 857.5 meters for the trip to the object and back. Divide that distance by two, and that gives a distance of 428.75 meters to the object that is reflecting the sound.
You could also divide the amount of time given initally by two. It took 2.5 seconds to make the trip to the object, reflect, and get back to the ship, so divide that time by two to get the time it takes for the sound to make it to the object. That would be 1.25 seconds. Take that amount of time and multiply it times the speed of sound, which is 343 meters per second, and you get the same distance as in the initial paragraph above, 428.75 meters.