If you throw a rock into a water this will create a wave. The wave would travel outward from the point where the rock hit the water. If propagation velocity is a constant, travelled distance will be velocity multiplied by time. Wavelength is distance travelled by wave for the time equals to wave period.
Thus, to find out wavelength, we need to multiply propagation velocity by period:
Using definition of frequency as
we can get standard wave relationship:
Calculator below computes unknown parameter using two known parameters.