\(\Leftrightarrow\left\{{}\begin{matrix}x>=-3\\21-x^2-4x=x^2+6x+9\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x>=-3\\x^2+6x+9-21+x^2+4x=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x>=-3\\2x^2+10x-12=0\end{matrix}\right.\Leftrightarrow x=1\)