A police car's radar gun emits microwaves with a frequency of f1 = 36.0 GHz. The beam

Question:

A police car's radar gun emits microwaves with a frequency of f1 = 36.0 GHz. The beam reflects from a speeding car, which is moving away at 43.0 m/s with respect to the police car. The frequency of the reflected microwave as observed by the police is f2.
(a) Which is larger, f1 or f2?
(b) Find the frequency difference f2 − f1.