`a) (x-3)^2-3x+9=0`
`<=> (x-3)^2-3(x-3)=0`
`<=> (x-3)(x-3-3)=0`
`<=> (x-3)(x-6)=0`
`<=>`\(\left[ \begin{array}{l}x-3=0\\x-6=0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=3\\x=6\end{array} \right.\)
`b) (x-1)(x^2+2x+1)`
`=x^3+2x^2+x-x^2-2x-1`
`=x^3+x^2-x-1`
`c) x(x^2-x+3)`
`=x^3-x^2+3x`