Ok so you have to use the first principle of derivates

f(x)=5

$\displaystyle \lim_{h\to \0}\frac{\f(x+h)-f(x)}{h}$

$\displaystyle \lim_{h\to \0}\frac{\5+h-5}{h}$

After this step everything cancels out and the derivative is equal to zero....but I don't think it's suppose to be equal to zero