3a+b/3a-b-3a-b/3a+b/[1/(1-b/3a)]/(1-2b/3a+b)
=(3a-3a)+(b/3a-b/3a)-b+b*(1-b/3a)/(1-2b/3a+b)
=-b+(1-1/3a)/(1/b-2/3a+1)
=-3/2+(1+2)/(2/3+4+1)
=-3/2+9/17
=-33/34