a+2=0
b-5=0
a=-2
b=5
3a²b-{2a²b-(2ab-a²b)-4a²}-ab
=3a²b-{2a²b-2ab+a²b-4a²}-ab
=3a²b-2a²b+2ab-a²b+4a²-ab
=4a²+ab
=4*4-2*5
=6