I want to align this equation. I tried
\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{fourier}
\usepackage[left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}
\begin{document}
If $U$, $V$, $W$, $u$, $v$, $v$, $w$ are lengths of edges of the tetrahedron (first three form a triangle; $u$ opposite to $U$ and so on),
\[\text{volume} = \frac{\sqrt {\,( - a + b + c + d)\,(a - b + c + d)\,(a + b - c + d)\,(a + b + c - d)}}{192\,u\,v\,w}\]
where
\begin{align*}
a &= \sqrt{xYZ}, & b = \sqrt{yZX},\\
c &= \sqrt{zXY}, &d = \sqrt{xyz},\\
X &= (w - U + v)\cdot(U + v + w), &x = (U - v + w)\cdot(v - w + U),\\
Y &= (u - V + w)\cdot(V + w + u), &y = (V - w + u)\cdot(w - u + V),\\
Z &= (v - W + u)\cdot(W + u + v), &z = (W - u + v)\cdot(u - v + W).
\end{align*}
\end{document}
I feel it is not good. How can I repair it?
&=
also in the equations on the right side