1/a-1/b=1
(b-a)/ab=1
b-a=ab
a-b=-ab
(a+3ab-b)/(3a-2ab-3b)
=(a-b+3ab)/(3a-3b-2ab)
=(a-b+3ab)/[3(a-b)-2ab]
=(-ab+3ab)/[-3ab-2ab]
=2ab/[-5ab]
=-2/5