1) 5x² - 5y² = 5(x² - y²) = 5(x - y)(x + y)
2) (5a + 6)² - 81 = (5a + 6)² - 9² = (5a + 6 + 9)(5a + 6 - 9) =
= (5a + 15)(5a - 3) = 5(a + 3)(5a - 3)
