The velocity of a wave is v, time period T, frequency n and wavelength λ.
By the definition of wavelength,
Wavelength = Distance travelled by the wave in one time period, i.e., in T second.
Or, wavelength = Wave velocity x Time period
Or, λ = v x T
Or, λ = v x 1/n [ T = 1/n]
Therefore, v = nλ
Therefore, Wave velocity = Frequency x wavelength