Answer
The hawk is approaching with a speed of 38 m/s
Work Step by Step
We can find the speed of the hawk as:
$f' = \frac{f}{(1-\frac{v_{source}}{v_{snd}})}$
$1-\frac{v_{source}}{v_{snd}} = \frac{f}{f'}$
$\frac{v_{source}}{v_{snd}} = 1-\frac{f}{f'}$
$v_{source} = (1-\frac{f}{f'})~ v_{snd}$
$v_{source} = (1-\frac{800~Hz}{900~Hz})~343~m/s$
$v_{source} = 38~m/s$
The hawk is approaching with a speed of 38 m/s.
