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\documentclass[12pt]{article}
\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath}
\begin{document}
\begin{gather*}
\begin{array}{rl}\begingroup
\renewcommand*{\arraystretch}{2.2}
\begin{array}{c}%equations
\dfrac{a}{b} = 0\\
\dfrac{c}{d} = 0
\end{array}
\endgroup
&
\begin{array}{rl}%variable explanations
a & $this is an a$\\
b & $this in a b$\\
c & $this is a c$\\
d & $this is a d$
\end{array}
\end{array}
\end{gather*}
\end{document}