How to draw a long line in math mode?

Question

As the question described, how to implement the picture below in LaTeX ? Which package should I use?

Answer 1

With less arbitrary alignments:

\documentclass{article}
\usepackage{amsmath}
\usepackage{chemformula}
\begin{document}
\begin{align}
& \ch{PuF6} + 2\ch{PuF3} = 3\ch{PuF4} \quad\
\quad-\quad \notag \
& \ch{F2} + 2\ch{PuF3} = 2\ch{PuF4} \[-1.2ex]
\cline{1-2}
& \ch{PuF6} = \ch{PuF4} + \ch{F2}
\end{align}
\end{document} 

Answer 2

Another solution with an alignat environment, chemformula and a \cmidrule:

\documentclass{article}
\usepackage{mathtools}
\usepackage{booktabs}
\usepackage{chemformula}
\usepackage{lipsum}
\begin{document}
{ \bfseries\boldmath\setlength{\aboverulesep}{-1.2ex}\setlength{\belowrulesep}{-0.5ex}
\begin{alignat}{2}
&\phantom{{}-{}} &\quad \ch{PuF6} + 2\ch{PuF3} & = 3\ch{PuF4} \
 \notag &{}- {}& & \
  & & \ch{F2} + 2\ch{PuF3} & = 2\ch{PuF4} \
  \cmidrule\heavyrulewidth{1-4}
 & &\ch{PuF6} & = \ch{PuF4} + \ch{F2}
\end{alignat}
}
\end{document} 

Answer 3

Something like this:

\documentclass{article}
\usepackage{mathtools}
\usepackage{booktabs, tabularx}
\usepackage[version=4]{mhchem}
\usepackage{lipsum}
\begin{document}
\begin{tabularx}{\linewidth}{X}
\begin{gather}
    \ce{PuF6} + \ce{2PuF3} = 3\ce{Pu}   \
\mathllap{-}

    \ce{PuF6} + \ce{2PuF3} = 3\ce{Pu}
\end{gather}                                 \[-5ex]
    \cmidrule1pt{1-1}        \[-8ex]
\begin{gather}
    \ce{PuF6} + \ce{2PuF3} = 3\ce{Pu}
\end{gather}
\end{tabularx}
\lipsum[1][1-3]
\end{document}