\(3x^2-2x\left(x+1\right)=-2x-18\)
=> \(3x^2-2x^2+2x=-2x-18\)
=> \(x^2=-2x-2x+18\)
=> \(x^2=18\)
=> \(\sqrt{x}=\sqrt{18}\)
=> \(\left|x\right|=3\sqrt{2}\)
=> \(\left[{}\begin{matrix}x=3\sqrt{2}\\x=-3\sqrt{2}\end{matrix}\right.\)
Vậy \(x\in\left\{3\sqrt{2};-3\sqrt{2}\right\}\)