\documentclass[]{slides}
\usepackage{epsfig,colordvi,color}

\begin{document}
% Subscript style
\newcommand{\subs}[1]{\mathrm{#1}}
\begin{center}
$G_0 \frac{B_0 + B_1 z^{-1} + B_2 z^{-2}}{1 + A_1 z^{-1} + A_2 z^{-2}}$
\end{center}
\end{document}
