Doppler Effect of a security alarm

[adashiu]

Homework Statement

The security alarm on a parked car goes off and produces a frequency of 735 Hz. The speed of sound is 343 m/s. As you drive toward this parked car, pass it, and drive away, you observe the frequency to change by 78.4 Hz. At what speed are you driving?

2. The attempt at a solution

Is it correct : ??

fo=735
f=735+78,4=813,4Hz

f=fo*[(V+Vo)/V]
v=36m/s

 

[Carid]

As you drive towards the stationary car the alarm frequncy is higher due to your movement.
As you go away from it the frequency is lower.
78.4Hz is the difference between these two frequencies.

Without actually doing the working it looks like you've found an answer twice the correct one.