Question

Obtain an expression for apparent frequency
of sound heard by a listener when both the
source and the listener are in motion and
approaching each other.

Answer 1

It's called Doppler effect
https://en.wikipedia.org/wiki/Doppler_effect
Total velocity of approaching of listener and source is V=Vlistener+Vsource
Formula for Doppler effect when source and listener approaching is
fapp=f0*(c+Vlistener)/(c-Vsource)
where
fapp - apparent frequency
f0 - frequency of source
c -speed of wave, in our case it's speed of sound = 331.2 m/s

Thus final expression is:
fapp=f0*(331.2+Vlistener)/(331.2-Vsource)