start with a form of the definition of a derivative at a point a
$f'(a) = \displaystyle{\lim_{x \to a}}\dfrac{f(x)-f(a)}{x-a}$
for the if direction assume there exists $\epsilon(x)$ that goes to 0 as $x \to a$ and show this leads to the above limit
for the only if direction assume the derivative exists, i.e. the above limit exists, and show the statement leads to requiring the limit of $\epsilon(x)$ to be as stated.