\(x^3-2x^2-5x+6=0\)
\(\Leftrightarrow x^3+x^2-2x-3x^2-3x+6=0\)
\(\Leftrightarrow x\left(x^2+x-2\right)-3\left(x^2+x-2\right)=0\)
\(\Leftrightarrow\left(x-3\right)\left(x^2+x-2\right)=0\)
\(\Leftrightarrow\left(x-3\right)\left(x-1\right)\left(x+2\right)=0\)
b/
\(x^3-x^2-8x+12=0\)
\(\Leftrightarrow x^3-4x^2+4x+3x^2-12x+12=0\)
\(\Leftrightarrow x\left(x^2-4x+4\right)+3\left(x^2-4x+4\right)=0\)
\(\Leftrightarrow\left(x+3\right)\left(x-2\right)^2=0\)