Answer
The official hears a frequency of $~~2000~Hz$
Work Step by Step
In this situation:
$f = 2000~Hz$
$v_D = 12~m/s$
$v_S = 12~m/s$
We can find the frequency that the official hears:
$f' = f~\frac{v-v_D}{v-v_S}$
$f' = (2000~Hz)~(\frac{343~m/s-12~m/s}{343~m/s-12~m/s})$
$f' = 2000~Hz$
The official hears a frequency of $~~2000~Hz$
