Equation broken into two (centered) lines with equation number in the middle of both lines

Question

I would like to know how to break a long equation into two lines and have both lines centered. Besides, I would like to have only one equation number in the middle of both lines.

Here's a MWE:

\documentclass{article}

\usepackage{amsfonts, amsthm, amsmath, amssymb}

\begin{document}



\noindent Text text text text text text text:
%
\begin{align}
123456789012345678901234567890123456789012345678901234567890=\\
123456789012345678901234567890123456789012345678901234567890\text{.}
\end{align}
%
Text text text text text text text.



\end{document}

I found something here: How to get only one vertically centered equation number in align environment with two equations.

In the link above the accepted answer explains how to have only one equation number in the middle of both lines. But I would like to have both lines centered, not aligned.

Answer 1

amsmath has documentation explaining a number of different environments and is worth a look. For example:

\documentclass{article}

\usepackage{amsfonts, amsthm, amsmath, amssymb}

\begin{document}

  \noindent Text text text text text text text:
  \begin{equation}
    \begin{split}
      123456789012345678901234567890123456789012345678901234567890=\\
      123456789012345678901234567890123456789012345678901234567890\text{.}
    \end{split}
  \end{equation}
  Text text text text text text text.

\end{document}

Answer 2

This uses the \Centerstack macro of stackengine. The distance between baselines is set with \setstackgap{L}{} to 14pt.

\documentclass{article}
\usepackage{amsmath}
\usepackage[usestackEOL]{stackengine}
\stackMath
\begin{document}
\noindent Text text text text text text text:
%
\begin{equation}
\setstackgap{L}{14pt}
\Centerstack{123456789012345678901234567890123456789012345678901234567890=\\
123456789012345678901234567890123456789012345678901234567890\text{.}}
\end{equation}
%
Text text text text text text text.
\end{document}