Plz help me out..ASAP..A 75-kHz radio transmitter sends an electromagnetic wave to a receiver L = 20.0 km away?

A 75-kHz radio transmitter sends an electromagnetic wave to a receiver L = 20.0 km away.The signal also travels to the receiver by another path where it reflects from a helicopter as shown. Assume that there is a 180° phase shift when the wave is reflected. (a) What is the wavelength of this EM wave? answer in... show more A 75-kHz radio transmitter sends an electromagnetic wave to a receiver L = 20.0 km away.The signal also travels to the receiver by another path where it reflects from a helicopter as shown. Assume that there is a 180° phase shift when the wave is reflected.

(a) What is the wavelength of this EM wave?
answer in km

please answer this question...10pts best answer..it would be greatly appreciated...thank you very much :)
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