A giao B bằng rỗng khi và chỉ khi:
\(\left[{}\begin{matrix}\left\{{}\begin{matrix}m\ge7\\m+3\le9\end{matrix}\right.\\m+3\le-3\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}m\ge7\\m\le6\end{matrix}\right.\\m\le-6\end{matrix}\right.\)
\(\Rightarrow m\le-6\)