Determine the value or values of ar for which the tangent to f is horizontal by first finding the derivative of f with respect to then solving f'(x)=0 for a PART 1. f(x) z +4 (2)-1 NOTE: for this problem, you should use the definition of derivative, f(a) = lim f(z)-f(x) 1-2 f'(x) lim 3-18 f(x+h)-f(x) h or the equivalent form PART 2. The tangent to the curve is horizontal at NOTE: Type answer in forma - c Separate multiple answers with a comma, such as z1,z=-1