An ambulance is traveling east at around 55.8 m/s. Behind it there is a car traveling all along the similar direction at 32.9 m/s. The ambulance driver hears his siren at a frequency of around 820 Hz. Determine at what frequency does the driver of the car hear the ambulance's siren. The velocity of sound in air is around 343 m/s. Answer must be in units of Hz.