Two cars are approaching each other at an equal speed of 7 . 2km hr...

Frequency of sound heard by car-$1,$ which comes by reflection from car-$2$

${f}_{1} = {f}_{0}\left(\dfrac{340 + 2}{340 - 2}\right)\left(\dfrac{340 + 2}{340 - 2}\right)$

$= {f}_{0}{\left(\dfrac{342}{338}\right)}^{2}$

Frequency of sound coming directly from car-$2$

${f}_{2} = {f}_{0}\left(\dfrac{340 + 2}{340 - 2}\right)$

$∴ {f}_{1} - {f}_{2} = {f}_{0}\left(\dfrac{342}{338}\right)\left(\dfrac{342}{338} - 1\right) = 8 . 09 ≃ 8Hz$