Step-by-step explanation:
Distance to wall = d
roundtrip to hear the echo = 2d
Distance = rate * time
2d = 343 m/s * 7.2 s
d = 1234.8 m
speed = wavelength * frequency
speed / wavelength = frequency
343 m/s / .87 m = 394 .25 Hz
period = 1/freq = .254 cm