Answer:
The sound takes time to go to a point and reflect back to your position.
It means the sound went twice a distance.
=> Suppose it takes about 0.5 seconds to hear the echo, the canyon wall would be [750 x (0.5/3600)]/2 = 0.052 miles
=> Suppose it takes about n seconds to hear the echo, the canyon wall would be [750 x (n/3600)]/2 miles
Hope this helps!
:)