Doppler effect! 45) A train moving at 31.8 m/s sounds a 136 Hz horn as it approaches a crossing. What frequenc

45) A train moving at 31.8 m/s sounds a 136 Hz horn as it approaches a crossing. What frequency does a driver waiting at the crossing hear?

I know this is a doppler effect equation, but there are so many variations I'm not sure which ones to use/what to do if I find the right one. HELP!

The observed frequency will be higher, because the train is approaching.
Using 343m/sec. as speed of sound,
136 x ((343 + 31.8)/343) = 148.61Hz.