\(\left(3x^2+x\right)^2+\left(3x^2+x\right)-2\left(3x^2+x\right)-2=0\)
\(\Leftrightarrow\left(3x^2+x\right)\left(3x^2+x+1\right)-2\left(3x^2+x+1\right)=0\)
\(\Leftrightarrow\left(3x^2+x-2\right)\left(3x^2+x+1\right)=0\)
\(\Leftrightarrow\left(x+1\right)\left(3x-2\right)\left[3\left(x+\dfrac{1}{6}\right)^2+\dfrac{11}{12}\right]=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x+1=0\\3x-2=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-1\\x=\dfrac{2}{3}\end{matrix}\right.\)