\(x^4-3x^3+2x^2+5x^3-15x^2+10x+6x^2-18x+12=0\)
\(\Leftrightarrow x^2\left(x^2-3x+2\right)+5x\left(x^2-3x+2\right)+6\left(x^2-3x+2\right)=0\)
\(\Leftrightarrow\left(x^2+5x+6\right)\left(x^2-3x+2\right)=0\)
\(\Leftrightarrow\left(x+2\right)\left(x+3\right)\left(x-2\right)\left(x-1\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x=-2\\x=-3\\x=2\\x=1\end{matrix}\right.\)