(10 points) Suppose that f(x)=6x+3x. (A) Find all critical values of f. If there are no critical values, enter-1000. If there are more than one, enter them separated by commas. Critical value(s) = (B) Use interval notation to indicate where f(x) is increasing. Note: When using interval notation in WeBWorK, you use I for co, -1 for-oo, and U for the union symbol. If there are no values that satisfy the required condition, then enter "0" without the quotation marks. Increasing: (C) Find the x-coordinates of all local maxima off. If there are no local maxima, enter -1000. If there are more than one, enter them separated by commas. Local maxima at x= (D) Find the x-coordinates of all local minima off. If there are no local minima, enter -1000. If there are more than one, enter them separated by commas. Local minima at x=