Consider the following function. f(x) = x-8x+7 (a) Find the critical numbers of f, if any. (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 ans does not exist, enter DNE.) increasing decreasing (c) Apply the First Derivative Test to identify all relative extrema. (If an answer does not exist, enter DNE.) relative minimum (x, y) = relative maximum (x, y) =