2(2A+B)-3(A+B)
=4A+2B-3A-3B
=A-B
=5a^3b+2a^4-3a^2b-ab^3
-(6ab^3-8a^3b+3a^4-5b^4)
=-a^4+13a^3b-3a^2b-7ab^3+5b^4.