A train whistle of frequency 264 Hz sounds higher or lower in pitch depending on whether it approaches or recedes.

A train whistle of frequency 264 Hz sounds higher or lower in pitch depending on whether it approaches or recedes.
Calculate the difference in frequency be- tween the approaching and receding train whistle for a train moving at a speed of 103 km/h. Take the speed of sound in air to be 340 m/s.
Answer in units of Hz.