Atruck is traveling at 74.5 kilometers per hour away from you. the driver is blowing the horn which has a frequency of 415 hz. the speed of the sound is 346 m/s. what is the observed frequency of the sound?

fo = 335 Hzlet fo = frequency heard 
f = actual frequency 
v= speed of sound 
vs = speed of source 

For the first one, 
fo = fv/(v - vs) 
74.5 km/h = 20.69 m/s 
vs = -20.69m/s since it is moving away from you 

fo = (415Hz)(346m/s)/(346m/s - (-20.69m/s)) 
fo = 392 Hz 

For the second one, 
fo = fv/(v - vs) 
82.8 km/h = 23 m/s 
vs = 23m/s 

fo = (312Hz)(331m/s)/(331m/s - (23m/s))