一个多项式加上3a的立方-10a的平方*b+b的立方=a的立方-3a的平方*b-5ab-2b的立方,求这个多项式.

a^3-3a^2b-5ab-2b^3-(3a^3-10a^2b+b^3)
=a^3-3a^2b-5ab-2b^3-3a^3+10a^2b-b^3
=a^3-3a^3+10a^2b-3a^2b-2b^3-b^3-5ab
=-2a^3+7a^2b-3b^3-5a