The problem is:

I already know that it will end as 10x (it's derivative by the way).

But using the difference quotient:

I'm told that,

Can be written in the form

Using the Calculus short cuts. I find that "B" is (10) and I find that "C" is (0)

However I don't know what what "A" is.

And when I do it long hand, I get stuck at this point.

How do I get "10x" from that above and find "A"?