Two loudspeakers are placed next to each other and driven by the same source at a frequency of 570 Hz.

Two loudspeakers are placed next to each other and driven by the same source at a frequency of 570 Hz. A listener is in front of the speakers and on on the line separating them, thus creating a constructive interference. What distance would one speakers be moved back away from the listener to produce destructive interference at the listener's ear? (speed of sound = 340m/s)