Answer
The driver of the car hears the siren at a frequency of $579~Hz$
Work Step by Step
We can use the equation for the Doppler effect when both the observer and the source are moving:
$f_o = \left(\frac{v-v_o}{v-v_s}\right)~f_s$
$f_o = \left(\frac{343~m/s-28~m/s}{343~m/s-44~m/s}\right)~(550~Hz)$
$f_o = 579~Hz$
The driver of the car hears the siren at a frequency of $579~Hz$
