\documentclass[12pt]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\RequirePackage{amsmath,amssymb,latexsym}
\pagestyle{empty}
\begin{document}
$  (a_{22}-a_{11})p_{1}p_{2}+a_{21}(p_{1}^{2}-p_{2}^{2})=0  $
\end{document}
