Matrices in equations

The amsmath package provides an environment called pmatrix -- short for "matrix surrounded by (round) parentheses", I suppose -- which is perfectly suited for the job.

\documentclass{beamer}
\usepackage{amsmath,amssymb}
\begin{document}
\begin{frame}
\[
\det
\begin{pmatrix}
   \lambda & -1 \\
   q & \lambda+p
\end{pmatrix}
=0
\quad \Leftrightarrow \quad
\lambda^2+p\lambda+q=0
\]
\end{frame}
\end{document}