Verified answer

So if the speakers are both on x-axis, and we are observing interference only on the x-axis, AND they are in phase with each other

then obviously the answer is half the wavelength. That's the configuration where the crests of one soundwave meet the troughs of the other.

Wavelength is given by v / f, where v is the speed of sound at 20°C.

But speakers can have any other phase difference which will affect the answer.