Te question is missing some information. However, I'll try to help you out with the steps.
We have the relation between velocity, wavelength and frequency expressed as follows:
c = λμ
where:
c is the speed of the wave
λ is the wavelength of the wave
μ is the frequency of the wave
Substituting in the above equation, we can get the frequency of the wave which is the number of vibrations per units of time
We can then get the periodic time which is 1/frequency
Hope this helps :)