prevent brackets from breaking over span of lines due to newline and alignment latex
\documentclass{article}
\usepackage{amsmath}
% We "\right." and "\left." at the breaking points( // and &)
%to prevent break. "\right." close the bracket before the break.
%"\left." opens( or continues) the brackets in the newline.
% Note the full-stop in "\left." and "\right." in below example
\begin{document}
\begin{align}
J &= 2 \left[3x \right. \nonumber \\
&+ \left. 4 \right]
\end{align}
\end{document}