A cop car moving at $121\mathrm{.}44$ MPH is in hot pursuit of art thieves in a porsche moving at $101\mathrm{.}44$ MPH$.$ They have their sirens $($Frequency $824$Hz$)$ and lights on$.($a$)$ at what frequency do the cops hear the siren? $($b$)$ As the cop car is about to pass you $($on the sidewalk$)$ what frequency do you hear? $($c$)$ just after it passes you, what do you hear as the frequency of the siren? $($d$)$ at what frequency do the art thieves hear the siren?