what is the frequency the horns emit assuming T=20 degrees C( V=343m/s)?

Question

two automobiles are equipped with the same single-frequency horn. when one is at rest and the other is moving toward an observer at 15m/s, a beat frequency of 5.5Hz is heard

Answer 1

the A of french horns is 440 hertz.

you should calculate the wavelength of the wave that is seen taking into account that you move 15 m/s towards the wave ( when car is approaching and ! different longer wavelength when the car is going away from you )
the wavelength can be calculated back into frequency