Speakers A and B are vibrating in phase. They are directly facing each other, are 7.4 m apart, and are each playing a 72.0 Hz tone. The speed of sound is 343 m/s. On the line between the speakers there are three points where constructive interference occurs. What are the distances of these three points from speaker A?

Question

Speakers A and B are vibrating in phase. They are directly facing each other, are 7.4 m apart, and are each playing a 72.0 Hz tone. The speed of sound is 343 m/s. On the line between the speakers there are three points where constructive interference occurs. What are the distances of these three points from speaker A? 
m (nearest) 
m (middle) 
m (farthest)

Calculate wavelength. When the difference in distances from each speaker are multiples of wavelength, construtcive interference occurs. For instance, if the wavelenght were 1 meter, then when distance A (speaker A to the point) = distance B -+ n Lambda

n= 0, 1, 2, ... constructive interference occurs. The easy solution is in the center, n=0. I suspect you will find a +- 1 solution.