Question

A 71.4cm long tube has a 71.4cm long insert that can be pulled in and out. A vibrating tuning fork is held next to the tube. As the insert is slowly pulled out, the sound from the tuning fork creates standing waves in the tube when the total length L is 77.5cm, 126.0cm, and 174.5cm. What is the frequency of the tuning fork? Take the speed of sound in air to be 343m/s.