Atruck is traveling 70.0 kph away from you. the driver is blowing the horn, which has a frequency of 400 hz. the air temperature is 25ác. what is the observed frequency of the horn?

There are several information's already given in the question. Based on those information's the answer can be easily deduced.
Speed of the truck = 70 kph
                              = 19.44 m/s
Speed of sound = 343 m/s
Then
Observed frequency = 400 Hz ∗ 343 m/s/19.44m/s + 343 m/s
                                 = 378.54
                                 = 379 Hz