Doppler and electromagnetic waves

[gamesandmore]

A speeder is pulling directly away and increasing his distance from a police car that is moving at 23 m/s with respect to the ground. The radar gun in the police car emits an electromagnetic wave with a frequency of 6.0 109 Hz. The wave reflects from the speeder's car and returns to the police car, where its frequency is measured to be 316 Hz less than the emitted frequency. Find the speeder's speed with respect to the ground.

fo = fs * (1 +- vrel/c)

I am stressing out and can't get this at all

 

[member 553083]

.Answer: The speed of the speeder with respect to the ground is vrel = (fo - fs)c/fo = (6.0x109-316x106)c/6.0x109 = 4.98 x 10^2 m/s