#### Answer

(a) The frequency heard is 647 Hz
(b) The frequency heard is 559 Hz

#### Work Step by Step

(a) We can convert the speed to units of m/s
$v = (90~km/h)(\frac{1000~m}{1~km})(\frac{1~hr}{3600~s})$
$v = 25~m/s$
We can find the frequency heard by a stationary observer in front of the car.
$f' = \frac{f}{(1-\frac{v_{source}}{v_{snd}})}$
$f' = \frac{600~Hz}{(1-\frac{25~m/s}{343~m/s}~)}$
$f' = 647~Hz$
The frequency heard is 647 Hz
(b) We can find the frequency heard by a stationary observer behind the car.
$f' = \frac{f}{(1+\frac{v_{source}}{v_{snd}})}$
$f' = \frac{600~Hz}{(1+\frac{25~m/s}{343~m/s}~)}$
$f' = 559~Hz$
The frequency heard is 559 Hz