### Physics: Speakers A & B are vibrating in phase; what is the distance?

Speakers A and B are vibrating in phase. They are directly facing each other, are 7.80 m apart, and are eack playing a 73.0-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 distance of these three points from speaker A?