A+B=(3x^3 - 6xy+7y^2)+(3x^2 +4xy- 2y^2)
=3x^3 - 6xy+7y^2+3x^2 +4xy- 2y^2
=3x^3+(-6xy+4xy)+(7y^2-2y^2)+3x^2
=3x^3-2xy+5y^2+3x^2
A-B=(3x^3 - 6xy+7y^2)-(3x^2 +4xy- 2y^2)
=3x^3 - 6xy+7y^2+3x^2-4xy+ 2y^2
=3x^3+(-6xy-4xy)+(7y^2+2y^2)+3x^2
=3x^3-10xy+9y^2+3x^2
B-A=(3x^2 +4xy- 2y^2)-(3x^3 - 6xy+7y^2)
=3x^2 +4xy- 2y^2+3x^3+6xy-7y^2
=3x^2+(4xy+6xy)+(-2y^2-7y^2)+3x^3
=3x^2+10xy-9y^2+3x^3