What is speed of the van? Doppler Effect.?

A radar device emits microwaves with a frequency of 3.98e+09 Hz. When the waves are reflected from a van moving directly away from the emitter, the beat frequency between the source wave and the reflected wave is 552 beats per second. What is the speed of the van? (Note: microwaves, like all forms of electromagnetic radiation, propagate at the speed of light c = 3.00e+08 m/s.)