%% 
%%  Ein Beispiel der DANTE-Edition
%%  Mathematiksatz mit LaTeX
%%  3. Auflage
%%  Beispiel 09-18-1 auf Seite 192.
%%  Copyright (C) 2018 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%%  See http://www.latex-project.org/lppl.txt for details.
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[10pt]{exaartplain}
\pagestyle{empty}
\setlength\textwidth{170.40707pt}
\AtBeginDocument{\setlength\parindent{0pt}}
%StartShownPreambleCommands
\usepackage{amsmath}
\usepackage[customcolors,shade,markings]{hf-tikz}
\usetikzlibrary{calc}
%StopShownPreambleCommands
\begin{document}
\begin{equation*}
\left.\begin{array}{rl}
-2\cdot \tikzmarkin[mark at=0.93]{col 1}
      (0.05,-0.2)(-0.05,0.4)2&= -4 \\
-2\cdot 1&= -2 \\
-2\cdot 0\tikzmarkend{col 1} &= 0
\end{array}\right\} \text{\small
  \shortstack{Product increases\\ by 2 each time.}}
\end{equation*}
\begin{tikzpicture}[remember picture,overlay]
\coordinate (col-aa) at ($(col 1)+(1.3,-1.8)$);
\node[align=left,right] at (col-aa) {\small{Annotation}};
\path[-stealth,red,draw,use marker id] (col-aa) -| (0,0);
\end{tikzpicture}
\end{document}