Two cars moving in opposite directions approach each other with...

Due to Doppler’s effect, when both observer and source are moving towards each other, then, the frequency measured by the observer differs from the actual frequency generated by the source.

Thus, the apparent frequency, ${f}_{A} = f\left[\dfrac{v + {v}_{0}}{v - {v}_{s}}\right]$

$= 400\left[\dfrac{340 + 16.5}{340 - 22}\right]$

${f}_{A} = 448Hz$