I can help you do them for yourself :)
Answer: 1000 Hz
Explanation:
You can calculate frequency by dividing velocity by wavelength
Frequency = velocity/wavelength
Find velocity first.
900 m/3 s = 300 m/s
Plug values in to find frequency.
F = (300 m/s)/0.3 m
F = 1000 Hz
Answer:
a) 42 m/s, positive direction (to the east), b) 42 m/s, negative direction (to the west).
Explanation:
a) Let consider that Car A is moving at positive direction. Then, the relative velocity of Car A as seen by the driver of Car B is:

42 m/s, positive direction (to the east).
b) The relative velocity of Car B as seen by the drive of Car A is:

42 m/s, negative direction (to the west).
Answer:
im pretty sure it B but I recommend waiting for another person. I used the workdone formula (Force*Dictance*cos(theta) and got 55 Joules
Explanation: