Interference with two speakers?

[TheTourist]

Interference with two speakers??!

Two speakers placed 1m apart emit sound of frequency f=1000Hz in phase. A listener walks along a line that is parallel to the line joining the two speakers and 8m from their midpoint. What is the distance he has to walk from the central maximum in loudness to the first minimum in loudness? use 340m/s for the speed of sound

I have done similar questions to this with no problem, but this question has completely stumped me and I don't know where to start.

Thanks

 

[willem2]

what can you say about the phase of the sounds received at the point where there is a minimum? what does this mean for the distances to the speakers?