Find f(a), f(a+h) and the difference quotient (f(a+h)-f(a))/ h, where hod want equal 0.
f(x)= 2x/ x-1

First. 

Calculate f'(x) = (2x/x-1)' = (-2)/; when x is not 1;

Second.

Calculate f'(0) = -2 => the difference quotient (f(a+h)-f(a))/ h, where hod want equal 0 is -2;

Third.

f(a) = 2a/a-1; f(a+h) = (2a+2h)/(a+h-1);