Determine the intervals on which the graph of y = f(x) is concave up or concave down, and find the x-values at which the points of inflection occur. f (x) = x(x - 9/x), x>0 (Enter an exact answer. Use symbolic notation and fractions where needed. Give your answer in the form of a comma separated list, if necessary. Enter DNE if there are no points of inflection.) X = (Use symbolic notation and fractions where needed. Give your answers as intervals in the form (*, *). Use the symbol . for infinity, U for combining intervals, and an appropriate type of parenthesis "(", ")", "[", or "]", depending on whether the interval is open or closed. Enter 0 if the interval is empty.) f is concave up when x E f is concave down when x E