While blowing its horn of f=524 Hz a car accelerates at 0.8m/s^2

the car starts from rest by your side and moves away. Find the time it taked for the frequency you hear to decrease by 5%.
Hint: Don't forget to include the time it takes for sound to reach you

any help would be great.

You want to know the car's velocity when it emits a sound frequency that is HEARD as 0.95*524 = 497.8 Hz

You will need to assume a sound velocity a, and that requires assuming an air temperature.

The received frequency is 497.8 Hz when
497.8 = 524[a/(a-V)]
(That is the Doppler formula fr a moving source).

Solve for the car velocity V when that sound is emitted.

Calulate the time the car has that velocity, using the acceleration.

Add the time it takes the sound to reach the listener. You will need the car's location when the sound was emitted.