I already have the answers, but I need the explanation

Q11: A police car in hot pursuit goes speeding past you. While the siren is approaching, the frequency of the sound you hear is 5500 Hz. When the siren is receding away from you, the frequency of the sound is 4500 Hz. Use the Doppler formula to determine the velocity of the police car. Use vsound=330 m/s. (10 points) What is the velocity v of the police car? 34 m/s Q12: Three polarizing filters are stacked with the polarizing axes of the second and third filters oriented at 30 and 60, respectively, relative to the polarizing axis of the first filter. Unpolarized light of intensity Io is incident on the first filter. Transmission Orientation Orientation (A) What is the intensity of light emerging from the first filter (1/) compared with lo? (3 points) 50% (B) What is the intensity of light emerging from the second filter (12) compared with lo? (3 points) 37.5% (C) What is the intensity of light emerging from the third filter (13) compared with lo? (4 points) 28.1%