((1/x)-(1/a))/(x-a) how to solve
what do you mean by "solve"? It's just an expression, equal to
-1/(ax)
However, it is the difference quotient for f(x) = 1/x. It is
(f(x)-f(a))/(x-a)
If you take the limit as x->a you get
-1/a^2
which happens to be df/dx at x=a.