[1/4 Points] DETAILS PREVIOUS ANSWERS TANAPMATH/ Effect of Advertising on Hotel Revenue The total annual revenue R of a certain resort hotel is related to the amount of money x the hotel spends on advertising its services by the function R(x) = -0.005x3 + 1.5x2+ 4x + 7,000 (0 s x 5 400) where both R and x are measured in thousands of dollars. (a) Find the interval where the graph of R is concave upward. (Enter your answer using interval notation.) Find the interval where the graph of R is concave downward. (Enter your answer using interval notation.) What is the inflection point of the graph of R? (x, R(x) ) =