Suppose that a police car is moving to the right at 27 m/s, while a speeder is coming up from behind at a speed of 39 m/s, both speeds being with respect to the ground. Assume that the electromagnetic wave emitted by the police car's radar gun has a frequency of 8.0 × 109 Hz. Find the difference between the frequency of the wave that returns to the police car after reflecting from the speeder's car and the original frequency emitted by the police car.