7. While standing at a crosswalk, you hear a frequency of 560 Hz from an approaching police car. Later, when you and the police car are both at rest, you note that the frequency of the siren is 517 Hz. What was the speed of the police car as it originally passed you by (assume that the temperature is at 20C)?
a. 13.1 m/s. b. 17.4 m/s. c. 21.1 m/s. d. 28.8 m/s. e. 26.3 m/s.