The formula f'=\(\frac{v}{v±vs}\)f does not hold if the observer is moving too
Where vo represents the velocity of the observer
If the observer is moving away from the source, we use -vo
If the observer is moving towards from the source, we use +vo
Don't get vs and vo mixed up. Vs is the velocity of the source of the sound
You will only use this equation for more difficult examples
For example, if the source is moving at 20 km/h, and the observer is moving at 10 m/s...
And always check if the frequency is in Hz or kHz.