Answer:
The difference in distance from the speakers is 5.2 - 3.5 = 1.7 m
The listener would be 1/2 wavelength out of phase with the speakers
1/2 y = 1.7 m where y is the wavelength
y = 3.4 m the required wavelength
f = v / y = 340 m/s / 3.4 m = 100 / sec lowest frequency
