\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
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\begin{document}
\begin{equation*}
  y(t) = \underbrace{C e^{At} \Bigl( x(0)-(s I-A)^{-1} B
           \Bigr)}_{\text{transient}} + 
           \underbrace{\Bigl( D+C(s I-A)^{-1}B \Bigr) e^{s
           t}}_{\text{steady state}},
\end{equation*}
\end{document}