\(x^3+x^2+2x-3x^2-3x-6=9\)
\(\Leftrightarrow x\left(x^2+x+2\right)-3\left(x^2+x+2\right)=0\)
\(\Leftrightarrow\left(x-3\right)\left(x^2+x+2\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x-3=0\\x^2+x+2=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=3\\\left(x+\frac{1}{2}\right)^2+\frac{7}{4}=0\left(vn\right)\end{matrix}\right.\)