Doppler effect with moving source and observer

[hopelessphysics]

Homework Statement

As you drive at a speed of 15.0 m/s an ambulance approaches you from behind with its siren blaring at a frequency of 1022 Hz. After the ambulance has passed you the signal is now 981 Hz. If both you and the ambulance travel at a constant speed how fast was the ambulance traveling?

Homework Equations

fo=fs(1+- vo/v)/(1+-vs/v)

The Attempt at a Solution

So I tried to use the equation above by plugging in the frequencies, but was trapped because the frequencies show only what the source was emitting, and not what the observer was hearing.

 

[mfb]

fo is the observer frequency.
You don't have the source frequency, but you have a second equation you can use.

 

[hopelessphysics]

What is the second equation?

 

[mfb]

The ambulance before and after it passes you => 2 equations