Final answer:
Tripling the wavelength of a wave – from 18.0 mm to 54.0 mm, with the original frequency of 6.0 Hz – reduces the frequency by a factor of three, resulting in a new frequency of 2.0 Hz.
Step-by-step explanation:
If a wave with a wavelength of 18.0 mm has a frequency of 6.0 Hz and the wavelength is tripled, then the new frequency can be calculated using the wave speed equation v = λf, where v is the wave speed, λ is the wavelength, and f is the frequency. Since wave speed remains constant for a given medium, tripling the wavelength means the frequency must be reduced by a factor of three to maintain the same speed. Hence, the new frequency will be 6.0 Hz / 3 = 2.0 Hz.