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](l{-0.6em}r{-1em}){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]
    \cmidrule[1pt](l{9em}r{9em}){1-1}        \\[-8ex]
\begin{gather}
    \ce{PuF6} + \ce{2PuF3} = 3\ce{Pu}
\end{gather}
\end{tabularx}

\lipsum[1][1-3]
\end{document}