Determine the value or values of x for which the tangent to f is horizontal by first finding the derivative of f with respect to x then solving f'(x) = 0 for x.
NOTE: for this problem, you should use the definition of derivative, f'(x) = lim_{h->0} (f(x+h) - f(x))/h or the equivalent form f'(x) = lim_{z->x} (f(z) - f(x))/(z - x).
PART 2.
The tangent to the curve is horizontal at: