The key to this one is that the observer needs to be standing at a point of total destructive interference. That is, the difference in the path length of the person to speaker A and the person to speaker B must be 1/2 wavelength.Originally Posted bypocahontas

So say the person is standing in front of speaker A. Call the distance between the speakers d and the distance from the person to each speaker a and b, respectively. These three distances make a right triangle. We know a = 12.0 m and we know b - a = 1/2 wavelength. (We know it is this and not a multiple of 1/2 lambda because we are looking for the minimum separation of the speakers.) From this you should be able to construct a value for b and thus, since you have a right triangle, figure a value for d.

-Dan