does a and b equal to?

So I understand the overall concept, and did f(5+h) = 1/(14+h) and f(5) = 1/14.

Then to subtract the two, you get the common denominator which is 14(14+h).

So when you subtract it becomes (14-14+h)/[14(14+h)].

And then you divide that by h and get h/(196h + 14h^2), which can be simplified to 1/(196+14h).

My question is why my answer 1/(196+14h) is positive and not in the requested format −1/(ah+b)?

What did I do wrong?

please explain and thanks