Two audio speakers are kept some distance apart

Two audio speakers are kept some distance apart and are driven by the same amplifier system. A person is sitting at a place $6.0 \mathrm{~m}$ from one of the speakers and $6.4 \mathrm{~m}$ from the other. If the sound signal is continuously varied from $500 \mathrm{~Hz}$ to $5000 \mathrm{~Hz}$, what are the frequencies for which there is a destructive interference at the place of the listener? Speed of sound in air $=320$ $\mathrm{m} \mathrm{s}^{-1}$.