Tìm x biết (x – 5)(x + 5) + 3(3 – x) = –11 + 10x.

x² – 25 + 9 − 3x + 11 − 10x = 0

⇔ x² − 13x – 14 = 0

⇔ x² + x − 14x – 14 = 0

⇔ x(x + 1) − 14(x + 1) = 0

⇔ (x + 1)(x − 14) = 0

⇔ \(\left[ \begin{array}{l}x + 1 = 0\\x - 14 = 0\end{array} \right.\)

⇔ \(\left[ \begin{array}{l}x = - 1\\x = 14\end{array} \right.\)

vậy x = –1 hoặc x = 14