Doppler Effect involving two moving objects

[sloane729]

Homework Statement

A truck moving at 36 m/s is overtaken by a police car moving at 45 m/s in the same direction. If the frequency of the siren relative to the police car is 500 Hz, what is the frequency heard by an observer in the truck as the police car approaches the truck? (The speed of sound in air is 343 m/s.)

Homework Equations

f'=(v+v_o)/(v-v_s)f

v_o = velocity of the truck
v_s = velocity of the police car

The Attempt at a Solution

I thought this would be a simple substitution problem but it's obviously not if I'm getting a different a wrong answer

f' = (343+36)/(343-45)(500) = 636Hz (This is my answer which was marked wrong)

Please advise

 

[olivermsun]

Think through the signs. The two vehicles do not have 36+45 m/s relative speed.