\(6\left(x+1\right)^2-2\left(x+1\right)^3+2\left(x-1\right)\left(x^2+x+1\right)=8\)
\(\Leftrightarrow6.\left(x^2+2.x.1+1^2\right)-2\left(x^3+3.x^2.1+3.x.1^2+1^3\right)+2.\left(x^3-1^3\right)=8\)
\(\Leftrightarrow6x^2+12x+6-2x^3-6x^2-6x-2+2x^3-2=1\)
\(\Leftrightarrow6x+2=1\)
\(\Leftrightarrow6x=-1\)
\(\Leftrightarrow x=-\frac{1}{6}\)