18. Consider the polynomial function f (x) = x4 - 3x3 - 19x2 + 21x + 84. a) Use the Intermediate Value Theorem to show that f (x) has at least one real zero between x = 2 and x = 3. b) Find f (-4) and f (0). What does the Intermediate Value Theorem tell us about the existence of zeros over the interval [-4,0]