Two speakers are driven by a common oscillator at 800 Hz and face each other at a distance of 1.25 m. Locate the points along a line joining the two speakers where relative minima would be expected.(Use v = 340 m/s).

Well, we know that identical sine waves originate at the two speakers. We also know that the sound waves propagate out from each speaker toward each other at a known velocity. At any given point between the two speakers, we can calculate the phase of the respective waves relative to their points of origin. The relative minima will occur at the point(s) where the phase difference between the two waves is 180 degrees (as well equivalent higher-order angles such as 540 degrees, 900 degrees, etc.) so that you get maximum destructive cancellation of the waves at those points.

We can use

v=f\cdot \lambda

to calculate the wavelength of the 800Hz sound waves, \lambda=0.425m. Thus we know that for every 42.5cm of distance we move away from one of the speakers, the sound wave will have traversed through 360 degrees of phase. Knowing that, you can then calculate the phase at any distance away from the speaker as

\phi=360\cdot \frac{d}{\lambda}

Now you just need to use that relationship to calculate the phase difference between the two waves at any point between the speakers and solve to find the places where the phase difference is +/-180, +/-540, etc.

Can you take it from there?