% !TEX TS-program = latex

\documentclass[11pt]{amsart}
\usepackage{geometry}                % See geometry.pdf to learn the layout options. There are lots.
\geometry{letterpaper}                   % ... or a4paper or a5paper or ... 
\usepackage{array}
\title{Tabl test}

\begin{document}
\begin{equation*}
	\begin{array}{cccc}
		\fbox{1}&R&2S+R&R\\
		&\fbox{2}&0&S\\
		&&\fbox{3}&-S+R\\
		&&&\fbox{4}
	\end{array}
\end{equation*}

\end{document}