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