Determine where the function is concave upward and where it is concave downward. (Round your answers to three decimal places, if necessary. Enter your answer using interval notation. If an answer does not exist, enter DNE.) f(x) = 9x2 + 4x1/2 + 16 Concave upward: Concave downward: Complete the following parts. (a) To determine the concavity of the function f, we need the critical numbers of -Select- ] (b) These critical numbers are as follows. (Enter your answers as a comma-separated list. If an answer does not exist, enter DNE.) X =