Destructive interference of sound waves

1. The problem statement, all variables and given/known data

Two loudspeakers are located 3.42m apart on an outdoor stage. A listener is 18.80m from one and 19.60m from the other. During the sound check, a signal generator drives the two speakers in phase with the same amplitude and frequency. The transmitted frequency is swept through the audible range (20 Hz to 20 kHz). What are the three lowest frequencies at which the listener will hear a minimum signal because of destructive interference.

2. The attempt at a solution

I know that (2n+1)/2 * wavelength = 19.6 - 18.8. So I try different values of n to find the wavelength then from this I try to calculate the frequencies. But I don't understand what the distance between the two speakers has to do with the problem.

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