\documentclass{standalone}

\usepackage{amsmath}

\begin{document}
\[
    \begin{vmatrix}
       a_{1,1}  & a_{1,2}  & a_{1,3} 
       \\
       a_{2,1}  & a_{2,2}  & a_{2,3} 
       \\
       a_{3,1}  & a_{3,2}  & a_{3,3} 
       \\
    \end{vmatrix}
\]
\end{document}
