Relation between a photon's energy and momentum

[Fluxthroughme]

For part b), I can see that, as $p = \frac{E}{c}$, then a photon with twice the momentum must have twice the energy. However, I cannot see the answer to part a). My book states that $k_2 = 4k_1$, but I would have thought that, as per the answer to b), the KE is just twice that of the other, not four times?

 

[f95toli]

The particle in part A is a proton, not a photon.
And since it traveling at a speed much lower than c you can just use classical mechanics.

 

[Fluxthroughme]

The particle in part A is a proton, not a photon.
And since it traveling at a speed much lower than c you can just use classical mechanics.

Oh wow... All this reading about photons has me seeing the word everywhere -_-

Thank you very much :)