Previous Problem Problem List Next Problem (1 point) Consider the function f(x) = 9(x - 4) 213. (a) Find all critical numbers coff. c = 4 (b) f is increasing for x E (c) f is decreasing for x E (d) f is concave up for x E (e) f is concave down for x E Note: Input U, infinity, and -infinity for union, co, and -co, respectively. If there are multiple answers, separate them by commas. If there is no answer, input none