Đáp án:
`x² - mx + 3m - 9 = 0`
`⇔ (x² - 3²) - m(x - 3) = 0`
`⇔ (x - 3)(x + 3) - m(x - 3) = 0 `
`⇔ (x - 3)(x + 3 - m) = 0`
`⇔` \(\left[ \begin{array}{l}x - 3 = 0\\x + 3 - m = 0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=3\\x=m=-3\end{array} \right.\)
$#lam$