if f(x)= X(squared)+2x+5

Then f(x+h) = ?

Just plug x+h in for x in your function and simplify.

(x+h)^{2}+2(x+h)+5

Do you have to go further and find the derivative via the definition?

That is, \lim_{h\to 0}\frac{f(x+h)-f(x)}{h}

Thanks!