%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-4-27 on page II-174 in "amsmath, mathtools, empheq --- Boxed formulas".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{159.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.3\abovedisplayskip}%
    \setlength\belowdisplayskip{.3\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{mathtools}
%StopShownPreambleCommands

\begin{document}
\begin{align}
  \Aboxed{ f(x) & = \int h(x)\, dx} \\
                & = \boxed{g(x)}
\end{align}
\end{document}