Waves and Sound: Find Moving Train Speed

[harujina]

Homework Statement

Two train whistles have frequencies of 180 Hz in air (v = 344 m/s). When one train is at rest sounding its whistle, a beat frequency of 2 Hz is heard from the moving train which is sounding its whistle as well. What is the moving train's speed if it is approaching the station?

Homework Equations

f_beat = l f_o - f₁ l

f_o = f_s (v + v_o / v - v_s) ; [source is approaching the observer]

f_o: observer
f_s: source

The Attempt at a Solution

v = 344 m/s
f_s = 180 Hz
f_o = 182 Hz [f_o = f_beat + f_s; = 2 Hz + 180 Hz]
v_o = 0
v_s = ?

f_o = f_s (v + v_o / v - v_s) ; [source is approaching the observer]

I'm trying to rearrange this formula to isolate v_s but for some reason I can't.
Where do I start... I've tried everything.

And sorry about all the questions, I'm having some significant trouble with this unit in physics...

 

[pbuk]

The first step is to multiply both sides by (v - v_s). Can you see why? What is the next step?