I have been trying to figure out the solution to this problem of finding the "velocity" of de-Broglie's wave.  I have tried to see answers from countless sources but none of them helped. My book provides me with the solution that : ***v = (h*f)/(m*c)*** . 
Where m is the mass of particle , f is the frequency, h is Planck constant and c is speed of light.

And this came from the fact that they used the formula **c = f×(lambda)** and substituted it in de-Broglie's equation for wavelength of a particle **(= h/p)** , which is really absurd to me as to why does the **speed of light** come here? Shouldn't it be **v(wave)= f×(lambda)** . Now I've also read that it has something to do with **group velocity of matter waves** but none of the sources which I read really explained as to why speed of light should be at play here. Can anyone explain this? 

Thanks :)