I was reading this paper "The Solution of the Diophantine Equation $x^2+3y^2= z^2$". I have tried to prove it.

Why $\frac{(z-x)}{2}=3y_1^2$ and $\frac{(z+x)}{2}=y_2^2$ ?
How about this theorem?

Why $z-x=3y_1^2$, $z+x=y_2^2$, $x=\frac{(-3y_1^2+y_2^2)}{2}$ and $x=\frac{(3y_1^2+y_2^2)}{2}$ ?
Please help me. Thank you.
Paper >> http:// www.m-hikari.com/ija/ija-2014/ija-13-16-2014/abdelalimIJA13-16-2014.pdf