You are thinking that f(x) means f times x.  
Please get rid of that idea now. f(x) does NOT mean f times x.
The (x) is just a place holder for whatever is going to be
substited for x on the right side of the equation.

Here's an example just like yours that you can go be as a model:



Simplify .

Do it in stages.  Find f(x+h), then f(x+h)-f(x), then .


First find f(x+h

 

So



and since



So




Then finally find 

we divide that by h:



So



Edwin