Consider the equation below. (If an answer does not exist, enter DNE.) f(x) =x3-9x2-21x + 4 Find the interval on which f is decreasing. (Enter vour answer using interval notation.) (b) Find the local minimum and maximum values of f. local minimum value local maximum value (c) Find the inflection point. (x, y) = Find the interval on which f is concave up. (Enter your answer using interval notation.) Find the interval on which fis concave down. (Enter your answer using interval notation.)