%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-6 on page II-135 in "A single equation on several lines: no alignment".
%%
%% 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{183.0pt}
%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands

\begin{document}
\begin{multline}  \tag{2}
  \sum_{t \in \mathbf{T}} \int_a^t
    \biggl\lbrace \int_a^t f(t - x)^2 \,
            g(y)^2 \,dx \biggr\rbrace \,dy \\
  = \sum_{t \notin \mathbf{T}} \int_t^a
      \biggl\lbrace g(y)^2 \int_t^a
        f(x)^2 \,dx \biggr\rbrace \,dy
\end{multline}
\setlength\multlinegap{0pt}
\begin{multline}  \tag{2}
  \sum_{t \in \mathbf{T}} \int_a^t
    \biggl\lbrace \int_a^t f(t - x)^2 \,
            g(y)^2 \,dx \biggr\rbrace \,dy \\
  = \sum_{t \notin \mathbf{T}} \int_t^a
      \biggl\lbrace g(y)^2 \int_t^a
        f(x)^2 \,dx \biggr\rbrace \,dy
\end{multline}
\end{document}