\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts}

\thispagestyle{empty}
\begin{document}
\begin{equation}
  P(x)=\left\{
    \begin{array}{cc}
      \frac{1}{b-a}&a<x<b\\
      0&{\rm otherwise}
    \end{array}
  \right .\notag
\end{equation}
\end{document}
