For f(x ) = x4 - 2x2+ 6 find the following. (A) f'(x) (B) The slope of the graph of f at x = 3 (C) The equation of the tangent line at x = 3 (D) The value(s) of x where the tangent line is horizontal (A) f'(x) = (B) At x = 3, the slope of the graph of f is (C) At x = 3, the equation of the tangent line is y = (D) The tangent line is horizontal at x = (Use a comma to separate answers as needed.)