Answer
The interference is destructive.
Work Step by Step
The wavelength of the sound is $\lambda=\frac{v}{f}=\frac{344\frac{m}{s}}{860Hz}=0.4 m$.
Consider diagram 16.21 in the textbook. The difference in path lengths to the speakers is 13.4m-12.0m = 1.4 m. This is equal to $3.5 \lambda$.
Sounds from the speakers arrive at point P out of phase by 3.5 cycles, so the interference is destructive.
