\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
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\begin{document}
\begin{equation*}
  x_e = \begin{bmatrix} 0 \\ 0 \end{bmatrix}, \qquad
  x_e = \begin{bmatrix} k \\ 0 \end{bmatrix}, \qquad
  x_e = \begin{bmatrix} H_e^* \\ L_e^* \end{bmatrix}, 
\end{equation*}
\end{document}