DeBroglie wavelength based on kinetic energy

[Cocoleia]

Homework Statement

I am working on a problem similar to this one:

In this solution, I do not understand what mc is, can someone explain? Also, would I follow the same type of steps if I have the kinetic energy of a photon and I need it's wavelength?

Homework Equations

The Attempt at a Solution

 

[gneill]

In this solution, I do not understand what mc is, can someone explain?

Looks like a typo. It should also be $m_e$, the rest mass of the electron.

Also, would I follow the same type of steps if I have the kinetic energy of a photon and I need it's wavelength?

Photons have no rest mass. Setting it to zero in the equation in your problem yields p = K/c, where K is the energy of the photon. That's a valid result linking energy and momentum for a photon. To get at the wavelength appeal to the Planck relationship, $E = h \nu$.