Answer
The frequency of the sound that is reflected back to the detector is $~~0.195~MHz$
Work Step by Step
First we can find the frequency of the sound in the reference frame of the truck.
In this situation:
$f = 0.150~MHz$
$v_D = 45.0~m/s$
$v_S = 0$
We can find the frequency of the sound in the reference frame of the truck:
$f' = f~\frac{v+v_D}{v}$
$f' = (0.150~MHz)~(\frac{343~m/s+45.0~m/s}{343~m/s})$
$f' = 0.16968~MHz$
Then we can find the frequency of the sound that is reflected back to the detector.
In this situation:
$f = 0.16968~MHz$
$v_D = 0$
$v_S = 45.0~m/s$
We can find the frequency of the sound that is reflected back to the detector:
$f' = f~\frac{v}{v-v_S}$
$f' = (0.16968~MHz)~(\frac{343~m/s}{343~m/s-45.0~m/s})$
$f' = 0.195~MHz$
The frequency of the sound that is reflected back to the detector is $~~0.195~MHz$
