A siren at rest has a frequency f0. Standing at a crosswalk, a student hears a frequency f' from the siren in an approaching police car. After the car passes, the observed frequency is f". Let the speed of the siren (or the car) be Vc, and the speed of sound be Vs. If f' and f" are 570Hz and 400 Hz respectively, and the speed of sound in air is 340 m/s, find the speed of the car. Answer in units of m/s