Physics: Smallest possible value of frequency

The figure below shows an experiment in which the sound waves generated by a singer and having a single frequency are emitted by two speakers. These sound waves interfere at a listener located at point P. If the interference at point P is constructive, what is the smallest possible value of the frequency? (Assume x = 5.8 m, θ = 16°, and use 343 m/s as the speed of sound in air.)

Diagram:
http://tinypic.com/r/10gjr7c/5

I had the same question on my hw.

First, you have to figure out the value of the second speaker. you do this by using the angle and x value:
5.8/cos29=6.63

Then you subtract:
6.63-5.8=.831

Then solve:
343/.831= 412.76