To calculate the frequency of a wave when the speed is unknown and only the wavelength is given, you need an additional piece of information. The formula relating frequency (f), speed (v), and wavelength (λ) is:
f = v / λ
This formula shows that frequency is equal to the speed of the wave divided by its wavelength. However, without knowing the speed, you cannot determine the frequency with just the wavelength.
To calculate the frequency, you would need either the speed of the wave or another known quantity that relates to the speed, such as the medium in which the wave is propagating.