Đáp án:
$a, 10xy(x - y) - 6y(y - x) $
$= 10xy(x - y) + 6y(x - y) $
$= (10xy + 6y)(x - y) $
$= 2y(5x + 3)(x - y) $
$b, x² - 9 - 2(x + 3)² $
$= (x - 3)(x + 3) - 2(x + 3)² $
$= (x + 3)[(x - 3) - 2(x + 3)] $
$= (x + 3)(x - 3 - 2x - 6) $
$= (x + 3)(-x - 9) $