Answer
The frequency of the sound reflected back to the trumpeter is $~~494~Hz$
Work Step by Step
First we can find the frequency of the sound at the wall.
In this situation:
$f = 440~Hz$
$v_D = 0$
$v_S = 20.0~m/s$
We can find the frequency of the sound at the wall:
$f' = f~\frac{v}{v-v_S}$
$f' = (440~Hz)~(\frac{343~m/s}{343~m/s-20.0~m/s})$
$f' = 467~Hz$
Then we can find the frequency of the sound reflected back to the trumpeter.
In this situation:
$f = 467~Hz$
$v_D = 20.0~m/s$
$v_S = 0$
We can find the frequency of the sound reflected back to the trumpeter:
$f' = f~\frac{v+v_D}{v}$
$f' = (467~Hz)~(\frac{343~m/s+20.0~m/s}{343~m/s})$
$f' = 494~Hz$
The frequency of the sound reflected back to the trumpeter is $~~494~Hz$
