about 18,000 m
The sound waves are vibrations. In the problem it says that sound waves travel 6,000 m/s which means for every second the vibrations go 6,000 meters. The question asks about 3 seconds which is 1 second, 3 times. For the first second the vibrations go 6,000m. The next second they go another 6,000 m, and the third second they go another 6,000 m. 6,000 times 3 equals 18,000 m. Since the word problem says "about 6,000 m/s" I'd include about 18,000 m in my answer.
