a-b/b=2/3
a/b-1=2/3
a/b=5/3
a=5b/3
a/(b-3a)
=(5b/3)/(b-3*5b/3)
=-5/12
(3a+2b)/(2a-3b)
=(3*5b/3+2b)/(2*5b/3-3b)
=7b/(b/3)
=21