Derivation of Einstein's Energy, Mass and Momentum Relationship

Sorry for the long delay. I wanted to make my math not be unreadable and hence made the entire post on a google doc and took screenshots. The post, therefore, looks kinda unaligned. 

Do make sure to read the Lorenz factor derivation done in my previous post as this post starts off assuming that you know what the Lorenz factor, γ,  means and that you know what relativistic time/mass mean.

Hope this helps, I'll finally be getting into the astrophysics side of this (tachyons/ energy-momentum/ space-time graphs/ spacelike timelike as soon as I manage to figure that stuff out).

Thanks for reading!

Mehul