Hi, for the following question:

You are standing next to a train track when a train comes towards you at 28m/s sounding its 400Hz horn. What is the frequency you hear? The speed of sound at your location is 332m/s.

A) 472 Hz
B) 400 Hz
C) 338 Hz
D) 474 Hz

I realise that you can rule out B and C, however, my calculation gave the result of 437 Hz. I used the equation below:



And then substituted the frequency on the RHS as 400, velocity of the observer as 0, the velocity of the wave as 332m/s, the velocity of the source as being 28m/s.

Can anyone give me some guidance as to where I've gone wrong? Thanks in advance.