(a－b+3)的二次方－(b－2a)(－b+2a)
= [（a-b）+3]^2+(b-2a)^2
=(a-b)^2+6(a-b)+b^2-4ab+4a^2
=a^2-2ab+b^2+6a-6b +b^2-4ab+4a^2
=5a^2-6ab+2b^2+6a-6b