[x（x²y²-xy）-y（x²-x³y）]÷3x²y
=[x²y（xy-1）-x²y（1-xy）]÷3x²y
=x²y[（xy-1）-（1-xy）]÷3x²y
=x²y[2（xy-1）]÷3x²y
=2（xy-1)/3