Two loudspeakers, an oscillator and constructive interference at a point?

[Cade]

1. The problem statement, all variables and given/known data
Two loudspeakers placed X meters apart are driven in phase by an audio oscillator, whose frequency
range is 1300 Hz to 1800 Hz. A point P is located A meters from one loudspeaker and B meters from the
other. The speed of sound is 344 m/s. What is the frequency produced by the oscillator, for which
constructive interference of sound occurs at point P?

2. Relevant equations


3. The attempt at a solution
I don't know how to start.

[Delphi51]

For constructive interference, the distance from the two sources to P must be an equal number of wavelengths. Writing that as an equation would give you a start!