7a^3-6a^3b+3a^2b+3a^3+6a^3b-3a^2b-10a^3+3
=(7a^3+3a^3-10a^3)+(-6a^3b+6a^3b)+(3a^2b-3a^2b)+3
=0+0+0+3
=3
题中给出的条件a=2,b-负1/3是多余的,他的说法有道理.