a+b=0
b-1/3=0
b=1/3
a=-1/3
(5a²b-2ab²-3ab）-（2ab+5a²b-2ab²）
=5a²b-2ab²-3ab-2ab-5a²b+2ab²
=-5ab
=-5(-1/3)*(1/3)
=5/9