two cars are heading at each other with the same speed v. The horn of one (f=3000Hz) is blowing,and is heard to have a frequency of 3400Hz by the people in the other car.Find v if the speed of sound is 344m/s

Answer 1

Review the formulas presented here:
In your case the source moves the observer AND the observer moves toward the source. The true frequency in still air is fo = 3000. The received frequency is 3400.

The received frequency is
f = fo*(344+V)/(344-V)
(344+V)/(344-V) = f/fo = 11.333
Solve for V