It's time to get UNlost. You just have to sort through it.
1) Fix your notation:
These are NOT the same:
x+3/5 and (x+3)/5
Remember your Order of Operations.
2) Don't write things that make no sense.
You have: fx=(x+Dx)-fx/Dx
Notice how "fx" is on both sides. I'm sure you didn't mean that. Plus, there's and 'f' missing on the right -- f(x+Dx)?