\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
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\begin{document}
\begin{equation*}
  \begin{aligned}
    u &= k e + k_i \int_0^t e(\tau)\,d\tau + k_d \frac{de}{dt} \\
       &= -k(x - x_d) - k_i \int_0^t (x-x_d)\,d\tau - k_d (\dot x - \dot x_d)
  \end{aligned}
\end{equation*}
\end{document}