You'll get destructive interference if both waves are the same frequency but the peaks of one wave overlap the troughs of the other wave.
That can only happen if one wave has to travel (1/2 wavelength) farther than the other one to reach your ears. So we want to find the lowest frequency for which 52 cm is 1/2 of a wavelength ... the wavelength is 104 cm.
Frequency = (speed) / (wavelength)
Frequency = (344 m/s) / (104 cm)
Frequency = (344 m/s) / (1.04 meter)
Frequency = (344 / 1.04) per second
Frequency = 330.8 Hz .
