Calculating Doppler Shift for Radar Gun and Moving Ambulance

[mexqwerty]

A police car is traveling at a constant speed of 100. km/h with a radar gun which generates and detects radiowaves. The police car and the ambulance drive directly towards each other. The radar gun produces radiowaves with a wavelength of 0.0650 m.

What doppler shift (Δf = f′–f) is detected by the radar gun after the radiowave has been reflected by the ambulance?

I don't really know what to do. Do I use the relative speed between the police car and ambulance which is 180kmph? The right answer is 1540 Hz.

 

[Simon Bridge]

You have to walk yourself through the steps - the police car is acting as a transmitter of the original wave, and a receiver of the reflected wave, and it's moving.

Personally, since radio waves are light, I'd use the relative speed and the relativistic Doppler shift. F it were sound you may have to account for the medium.