The frequency of a car horn encountered a change from 400Hz to...

Wall as an observer
Frequency received by wall,

${f}_{1} = {f}_{0}\left(\dfrac{C}{C - V}\right)$
Again wall as a source
Frequency received by observer on car,
${f}_{2} = {f}_{1}\left(\dfrac{C + V}{C}\right)$
${f}_{2} = {f}_{0}\left(\dfrac{C + V}{C - V}\right)$
$500 = 400\left(\dfrac{C + V}{C - V}\right)$
$\dfrac{5}{4} = \dfrac{C + V}{C - V}$
$C = 9V$

$V = \dfrac{C}{9} = \dfrac{330}{9}m{s}^{- 1}$
$V = \dfrac{330}{9} \times \dfrac{18}{5} = 132km{h}^{- 1}$