\(x^4-2x^3-6x^2+16x-8=0\)
\(\Leftrightarrow x^4-2x^3-6x^2+12x+4x-8=0\)
\(\Leftrightarrow x^3\left(x-2\right)-6x\left(x-2\right)+4\left(x-2\right)=0\)
\(\Leftrightarrow\left(x-2\right)\left(x^3-6x+4\right)=0\)
\(\Leftrightarrow\left(x-2\right)\left(x-2\right)\left(x^2+2x-2\right)\)
\(\Leftrightarrow\left(x-2\right)^2\left(x^2+2x-2\right)=0\)