Two Police Cars Have Identical Sirens That Produce Sound of Frequency

Short Answer

Two police cars have identical sirens that produce sound of frequency of 570 Hz. A
stationary listener is standing between two cars. One car is parked and the other is
approaching the listener and both have their sirens on. The listener measures 9.0 beats per
second. Find the speed of the approaching police car. The speed of sound is 340 m/s.