First, since the echo frequency is greater, the car must be roaming
to the police car.
This is for the reason that the car is "running over" its own wave
fronts, creating the wavelengths tinier, and henceforth increasing the
frequency.
The trick here is to understand that the relative speed between the cars is
half of the speed of the echo for the reason that the echo is returning to the
police car at the similar speed as the sound wave trips to the moving
car.
It would also appear that the speed of sound has been expected to be 337
m/s
So if v is the moving car's velocity, the correct formula to use is:
2v / 337 = (1250/1200) - 1
2v = 14.04 m/s
v = 7.02 m/s