(a+b)² - 4
= (a+b)² - 2²
= (a + b - 2)(a + b +2)
(a-2ab)² - 4b²
= (a - 2ab)² - (2b)²
= (a - 2ab - 2b)(a - 2ab + 2b)
(a+3b)² - 9b²
= (a + 3b)² - (3b)²
= (a + 3b - 3b)(a + 3b + 3b)
= a(a+6b)
4a² - (a+b)²
= (2a)² - (a+b)²
= (2a-a-b)(2a+a+b)
= (a-b)(3a+b)