How Does Wind Affect the Doppler Effect in a Two-Train Scenario?

[LeFerret]

Homework Statement

Two trains are traveling toward each other at 30.0 m/s relative to the ground. One train is blowing a whistle at 480 Hz. (a)What frequency is heard on the other train if the wind is blowing at 30.0 m/s toward the whistle and away from the listener? (assume the speed of sound is 343m/s)

Homework Equations

F'=F((V+V_d)/(V-V_d))

The Attempt at a Solution

Well the top velocity has a + and the bottom has a minus because they're going towards each other and the wind is going away from the listener and towards the source.
So I just assumed F'=480((343+30+30)/(343-30+30)) since this is the vector sum? This is wrong however and it's actually the opposite, so I need help understanding why conceptually

 

[haruspex]

You need to take each speed relative to the medium.