\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
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\begin{document}
\begin{equation*}
  k_r = {-1}/\left(C (A-BK)^{-1} B\right).
\end{equation*}
\end{document}