(1 point) Let f(x) = 6x3 +90x2 - 7. ms At what x-values is f" (x) zero or undefined? X = (If there is more than one such x-value, enter a comma-separated list; if there are no such x-values, enter "none".) On what interval(s) is f(x) concave up? f(x) is concave up for x in (If there is more than one such interval, separate them with "U". If there is no such interval, enter "none".) On what interval(s) is f(x) concave down? f(x) is concave down for x in (If there is more than one such interval, separate them with "U". If there is no such interval, enter "none".)