Physics problem help?

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The railroad crossing lights turn red, so McKayla and her sister must stop and wait for the train to pass by. As they wait, McKayla's sister Kylie grabs her phone and uses an app to measure the frequency of the approaching train's horn. The app reads 429Hz. Assuming the train's original horn frequency is 400Hz and the speed of sound is 330m/s, how fast is the train going in m/s and miles per hour?
The equation is fo=[v/(v+vs)]*fs
fo= frequency heard by observer
fs = frequency emmitted by source of sound
v = speed of sound
vs = speed of sound source

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429 = [ 330 / (330 + vs) ] * 400
429 / 400 = 330 / (330 + vs)
330 + vs = 330 * 400 / 429
vs = 330 * 400 / 429 - 330
vs = -22,3 m/s
The minus sign indicates that the train is approaching her.