A+B=2x^2-6x+3 A-B=X^2-7x+5
则(A+B)-(A-B)=(2x^2-6x+3)-(x^2-7x+5)
A+B-A+B=2x^2-6x+3-x^2+7x-5
2B=x^2+x-2
B=x^2/2+x/2-1