A sound wave of frequency v travels horizontally to the right. It is reflected from a large vertical plane surface moving to left with a speed $u.$ The speed of sound in medium is $c.$ 

the number of waves striking the surface per second is $\frac{\left(c+u\right)}{c}v$

The wavelength of reflected wave is $\frac{c(c-u)}{v(c+u)}$

the frequency of the reflected wave heard by a stationary observer is $v\frac{\left(c+u\right)}{c}$

the number of beats heard by a stationary listener to the left of the reflecting surface is $\frac{2uv}{c-u}$