Use the makecell package
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\usepackage{makecell} % Use the makecell package
\begin{center}
\begin{tabular}{ | c | c | c |}
A Head & A Second Head & A Third Head\\
\hline
Some text & \makecell{Some really \\ longer text} & Text text text \\
\end{tabular}
\end{center}
https://tex.stackexchange.com/questions/2441/how-to-add-a-forced-line-break-inside-a-table-cell
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\begin{gather}
a + b = c \\
a = c - b
\end{gather}
%
With the align environment you can align equations:
%
\begin{align}
a + b &= c \\
a &= c - b
\end{align}