ĐK: \(-7\le x\le3\)
\(\sqrt{-x^2-4x+21}< x+3\)
\(\Leftrightarrow\left\{{}\begin{matrix}x+3>0\\-x^2-4x+21< x^2+6x+9\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x>-3\\2x^2+10x-12>0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x>-3\\\left(x+6\right)\left(x-1\right)>0\end{matrix}\right.\)
\(\Leftrightarrow x>1\)
\(\Rightarrow x\in(1;3]\)