(2a-b+3)(2a-3+b)=[2a+(3-b)]×[2a-(3-b)]=(2a）²-（3-b）²=4a²-(9-6b+b²)=4a²-9+6b-b²
(2a-b+3)(2a-3+b)={2a-(b-3)}{2a+(b-3)}=(2a)²-(b-3)²=4a²-b²+6b-9
