平方和绝对值相加为0则都等于0
所以a+2=0,3b-1=0
a=-2,b=1/3
原式=3a^2-2ab^2+6(ab-2分之1a^2b)-4ab-2ab
=3a^2-2ab^2+6ab-3a^2b-4ab-2ab
=3a^2-2ab^2-3a^2b
=12+4/9-4
=76/9