A truck moving at 39m/s passes a police car moving at 45 m/s in the opposite direction. If the frequency of the siren is 800 Hz relative to the police car, what is the frequency (in Hz) heard by an observer in the truck as the two vehicles approach each other? What is the frequency (in Hz) heard by an observer in the truck as the two vehicles left each other? (The speed of sound in air is 343 m/s.)