\(\Leftrightarrow\left[{}\begin{matrix}m^2+6m+9=4m-8\\m^2+6m+9=-4m+8\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}m^2+2m+17=0\\m^2+10m+1=0\end{matrix}\right.\)
\(\Leftrightarrow\left(m+5\right)^2=24\)
\(\Leftrightarrow\left[{}\begin{matrix}m=2\sqrt{6}-5\\m=-2\sqrt{6}-5\end{matrix}\right.\)