（a＋2）²+丨3b-1丨=0
所以a+2=0,3b-1=0
a=-2,b=1/3
所以原式=3a²b-(3ab²-6ab+3a²b+4ab)-2ab
=3a²b-(3ab²-6ab+3a²b+4ab)-2ab
=3a²b-(6ab²-2ab)-2ab
=3a²b-6ab²+2ab-2ab
=-3a²b
=-3×4×1/3
=-4