(x - 3y) + (x² - 9y²) = (x - 3y) + ( x - 3y)(x + 3y) = (x - 3y)(1 + x + 3y)
(9a² + 6ab + b²) - 25 = (3a + b)² - 5² = (3a + b - 5)(3a + b + 5)
ab⁵ - b⁵ - ab³ + b³ = (ab⁵ - b⁵) - (ab³ - b³) = b⁵(a - 1) - b³(a - 1) = b³(a - 1)(b² - 1)=
= b³(a - 1)(b - 1)(b + 1)