\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
	\begin{tikzpicture}[scale=0.5]
		\coordinate[label=above:$A$] (A) at (2,3);
		\coordinate[label=left:$B$] (B) at (-1, -2);
		\coordinate[label=right:$C$] (C) at (4,-1);
		\coordinate (D) at ($(A)+(C)-(B)$);		
		\draw (A)--(B)--(C)--(D)--cycle;
		\foreach \i in {A,B,C,D}
			\fill[red] (\i) circle (1.5mm);
		\path (D) node[right,blue]{$D$};
	\end{tikzpicture}
\end{document}