Answer
Please see the work below.
Work Step by Step
We know that according to the Doppler Effect, the change in frequency of a wave for an observer which moves relative to its source is given as
$f^{\prime}=f(1\pm \frac{u}{c})$. This equation shows that if the frequecy increases, then the system is approaching, and if the frequency is decreased then the system should be receding.
