+118667 Firstly your formula is not quite right, you need the extra brackets
[f (x +h) - f(x)] / h
f(x) = x^2 + 4x
[f (x +h) - f(x)] / h =?
f(x+h) = (x+h)^2+4(x+h)
Just write it all down and simplify.
If you have more problems with this then say so but try and explain what the problems are.
