=[2(a-3)]²-(a+3)²
=[2(a-3)+(a+3)][2(a-3)-(a+3)]
=(3a-3)(a-9)
=3(a-1)(a-9)
(a+b)²-(3a)²
=(a+b+3a)(a+b-3a)
=(4a+b)(b-2a)