原式=3a²b-2ab+1/3ab-3/2a²b
=(3-3/2)a²b+(-2+1/3)ab
=3/2a²b-5/3ab
当a=2, b=-1式
原式=3/2×2²×(-1)-5/3×2×(-1)
=3/2×4×(-1)+5/3×2×1
= -6+10/3
= -8/3