\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  g = \begin{bmatrix} R & p \\ 0 & 1 \end{bmatrix} \qquad
  \aligned
    R &\in SO(3) \\
    p &\in {\mathbb R}^3.
  \endaligned
\end{equation*}
\end{document}