What is the frequency of the siren's sound that the fire engine's driver hears reflected from the back of the truck?

show the wavelengths of a wave. show the crests of a wave.

i will answer your question if you answer mine

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me