Answer:
by reducing the frequency of the wave by a factor of three
Step-by-step explanation:
Speed = wavelength * frequency.
Wavelength = speed/frequency
The speed of an electromagnetic wave in a vacuum is constant. Meanwhile the wavelength and frequency have an inverse relationship which means for every rise in any of the parameters there will also be a corresponding fall.
Since the wavelength is tripled then the frequency will also be reduced by a factor of 3