12.9 An air-filled hollow rectangular waveguide is 150 m long
and is capped at the end with a metal plate. If a short pulse of
frequency 7.2 GHz is introduced into the input end of the guide,
how long will it take the pulse to return to the input end? Assume
that at the cutoff frequency of the guide is 6.5 GHz. Should I use
Phase Velocity or Group Velocity to solve this (Please explain
why)