A train whistles (f = 400 Hz) sounds higher or lower in frequency depending on whether it

Question:

A train whistles (f = 400 Hz) sounds higher or lower in frequency depending on whether it approaches or recedes.
(a) Prove that the difference in frequency between the approaching and receding train whistle is where u is the speed of the train and v is the speed of sound.
Δf = 2u / v / 1 ─ u2 / v2 f
(b) Calculate this difference for a train moving at a speed of 130 km/h. Take the speed of sound in air to be 340 m/s.