（1）-ax² + 6ax - 9a
= -a(x² - 6x + 9)
= -a(x² - 2*x*3 + 3²)
= -a(x - 3)²
(2) 4a² - 9(a - b)²
= (2a)² - [3(a - b)]²
= [2a + 3(a - b)] [2a - 3(a - b)]
= (5a - 3b)(-a + 3b)