How do I find the speed of this train?

[jesseeeeee]

Homework Statement

Two train whistles have an identical frequency of 246 Hz. Train A is motionless in the station and sounds its whistle. Meanwhile, Train B is pulling into the station and sounds its whistle. The conductor of Train B hears 5.39 beats every second. How fast is Train B moving? The speed of sound is 340 m/s.

Homework Equations

f' = f (v+0 / v-vs)

The Attempt at a Solution

 

[Simon Bridge]

Welcome to PF;
Please show an honest attempt at the problem ... what are the main physical principle and effect that is in play here?