14. [-/1 Points] DETAILS LARCALCET7 4.3.027. MY NOTES ASK YOU Consider the following function. f(x) = -5x3+ 15x +6 (a) Find the critical numbers of f. (Enter your answers as a comma-separated list.) X = (b) Find the open intervals on which the function is increasing or decreasing. (Enter your answers using interval notation. If an a not exist, enter DNE.) increasing decreasing (c) Apply the First Derivative Test to identify the relative extremum. (If an answer does not exist, enter DNE.) relative maximum (x, y ) = relative minimum (x, y ) = Submit