\documentclass{book}
\usepackage{graphicx}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{mathtools}
\usepackage{mathabx}
\usepackage{xepersian}

\begin{document}
\begin{center}
\textbf{آنگاه} 
\left\{
\begin{array}{ll}
\dot{x}=\left(\begin{array}{cc}
0 & 0\\
1 & -3\end{array}\right)x\\
y=x_{1}\end{array}
\end{center}\\

  

\end{document}