\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
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\begin{document}
\begin{equation*}
  \aligned
  m \ddot x &= -mg\sin\theta -c \dot x + u_1 \cos\theta - u_2 \sin\theta, \\
  m \ddot y &= mg(\cos\theta - 1) -c \dot y + 
    u_1 \sin\theta + u_2 \cos\theta, \\
  J \ddot \theta &= r u_1.
  \endaligned
\end{equation*}
\end{document}