Answer:
11.16 m
Explanation:
total distance traveled by the wave = speed * time
                              = 0.720 m/s * 31.0 s
                              = 22.32 m
Since this is an echo back to the starting point then the length of the swimming pool must be half of the distance traveled.
length of swimming pool  =  22.32 m÷2 = 11.16 m