where $v_{s}$ is the velocity of sound $w.r.t$ medium and $v_{m}$ is the velocity of the medium.

Thus $v^{\prime}=C+u$

As both the source as well as the observer are at rest, so frequency will remain the same as $f$ irrespective of the velocity of medium.

Let $\lambda$ be the wavelength of the sound wave detected by observer.

using, $v^{\prime}=f \times \lambda$

$C+u=f \lambda \Longrightarrow \lambda=\frac{C+u}{f}$