applying chain rule
f'(X)=a+b+2x*[(a+x)/(b+x)]a+b+2x-1*1/1
now putting x=0
f'(x) at =0 is a+b(a/b)a+b-1
applying chain rule
f'(X)=a+b+2x*[(a+x)/(b+x)]a+b+2x-1*1/1
now putting x=0
f'(x) at =0 is a+b(a/b)a+b-1
logf(x)=(a+b+2x)log[(a+x)/(b+x)]
differentiate it and get the answer