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\chapter{Prove that $\{6n+9 : n \in \bbz\} = \{6n-3 : n \in \bbz\}$.}

\begin{remark*}
To prove that two sets are equal, give a double containment argument. 
\end{remark*}

\section{First draft}
Due Monday, February 29 at 6:00 PM.

\begin{proof}

\end{proof}