(a+1)^2-9(a-1)^2=(a+1)^2- [3(a-1)]^2=[a+1+3(a-1)]*[a+1-3(a-1)]=(4a-2)(-2a+4)=2(2a-1)(2-a)(2a+1)^2-12(2a+1)+36=(2a+1)^2- 2*(2a+1)*6+6^2=[(2a+1)-6]^2=(2a-5)^2