documentclass{article}
usepackage{tikz,xcolor}
usetikzlibrary{positioning}

begin{document}

begin{tikzpicture}
filldraw[fill=gray!40!white, draw=black] (0,0) rectangle (4,2);
node[black](dot) at (2,1) {textbullet};
node[below=0cm of dot]  {small{$x_a = (2,1)$}};
end{tikzpicture}

end{document}