[fk378]

For the equation c=hv
where c=speed of light, h=Planck's constant, and v=frequency...

can u (speed of a particle/electron) be substituted in for c? ie, if given the frequency of an electron and we need to find its speed, can we use this equation (u=hv) or must we get to the answer by using other equations?

[Yggdrasil]

I think you're confusing some equations here.

The energy of a photon E is given by the equation

E = hν

and, the relationship between speed, wavelength and frequency of light is:

c = λν

The energy of a particle with mass can be derived from these particles as follows:

Special relativity tells us that E = pc for particles with no rest mass.  Therefore, since photons have no rest mass, we can rearrange this equation to find the momentum (p) of a photon:

p = E/c

Since E = hν and c = λν, we can rearrange to write the formula:

p = h/λ

which gives the momentum of a photon.  Conversely, we can solve for wavelength:

λ = h/p

and find an expression of the wavelength of a particle with mass.