To find the wavelength of two waves traveling in opposite directions from each other when their speed is known, you need to know either the frequency or the period of the waves. The relationship between wavelength (λ), frequency (f), and wave speed (v) is given by the equation:
v = λf
If you have the frequency (or period) of the waves, you can use the above equation to determine the wavelength.
However, if you only know the speed of the waves and not their frequency or period, you cannot directly determine the wavelength. The wavelength depends on the frequency, and without that information, it is not possible to calculate the wavelength.
In summary, you need either the frequency (or period) or the wavelength itself to determine the other quantity when the speed is known.