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