Replying to Help with physics problem!

This is a doppler effect problem, the equation you'll need to use is f'=f(Vsound+Vobserver/Vsound-Vsource)

When at rest, two trains have sirens that emit a frequency of 300Hz. The two trains travel toward one another and toward an observer stationed between them. One of the trains moves at 30 m/s, and the observer hears a beat frequency of 3 beats per second. What is the velocity of the second train, which travels faster than the first?