\(\Leftrightarrow\left(x+3\right)^2-m\left(\left|x+3\right|+1\right)-9\le0\)
Đặt \(\left|x+3\right|=t\ge0\)
\(\Rightarrow t^2-mt-m-9\le0\)
\(\Leftrightarrow t^2-9\le m\left(t+1\right)\)
\(\Leftrightarrow m\ge\frac{t^2-9}{t+1}\) (do \(t+1>0\))
Để BPT vô nghiệm \(\Leftrightarrow m< \min\limits_{t\ge0}\frac{t^2-9}{t+1}=-9\)
\(\Rightarrow m< -9\)