Q:

Is the relation c=νλ valid only for Electromagnetic waves?

What is the validity of the relation $c = \nu\lambda$?
More specifically, is this equation valid only for Electromagnetic waves?
I read this statement in a book, which says:

de Broglie waves are not electromagnetic in nature, because they do not arise out of accelerated charged particle.

This seems correct, but arises a doubt in my mind. 
Suppose I find out the wavelength of a matter wave (or de Broglie wave) using de Broglie's wave equation: $$\lambda = \frac{h}{p}$$
Now, can I use $c = \nu\lambda$ to find out the frequency of the wave?

A:

No. 
The general relation is given by
$$v = \lambda\nu$$
Where $v$ is the velocity of the considered wave and $\lambda$ and $\nu$ its wavelength and frequency. Of course in the case of an electromagnetic wave which is traveling at the speed of light you gain
$$c = \lambda\nu$$
If you're treating instead some massive particle, then thou have $$p = \frac{Ev}{c^2}$$ 
and using $E = \frac{h}{\nu}$ you obtain $$p = \frac{h}{\lambda}$$
For a non-relativistic particle 
$$\lambda = \frac{h}{p} = \frac{h}{\sqrt{2mK}}$$, where $K$ is the non-relativistic kinetic energy $K = \frac{p^2}{2m}$.

