$v=350 \,m / s \quad f=500 \,Hz$

$v=\frac{\omega}{k}$

$k=\frac{2 \pi f}{v}$

$k=\frac{2 \pi \times 500}{350}$

$k=\frac{2 \pi}{0.7}$

$\therefore \lambda=0.7 \,m$

Let equation of wave be

$y=A \sin (k x-\omega t)$

Let $x_1=0$ $x_2=1 \,m$

$y_1=A \sin (\omega t)$

$y_2=A \sin \left(\frac{2 \pi}{0.7} \times 1+\omega t\right)$

Hence phase difference $=\frac{20 \pi}{7}$ or approximately

Phase difference $\approx 3 \pi$

Which is same as $\Delta \phi=\pi$