Doppler Effect and a Motion Detector

A stationary motion detector sends sound waves of 0.120 MHz toward a truck approaching at a speed of 55.0 m/s. assume the speed of sound is 343.0 m/s
What is the frequency of the waves reaching the detector after reflecting on the truck?

f' is given by f'=[ (V+U) / (V-U ) ](f)
where F' is the apparent freq, V=343 m/s u=55 m/s and f=freq of source

so substituting we get 0.1658 MHz