need help with the correct answer

Problem. (4x25=100pts) The revenue R(x) (in thousands of dollars) generated from sales of a certain product is related to the amount x spent (in thousands of dollars) on advertising by 4 R(x) (-x} + 66x2 + 1050x 400), 27 Osxs 25. a) Find the inflection point, if exists. b) By using the second derivative test, find the intervals where the revenue function R(x) is concave upward and concave downward. c) Determine the point of diminishing returns. Explain why this point is the point of diminishing returns. (You can use the results found in parts a and b for your explanation) d) Maximum how much money spent on advertising could be considered a good use of capital. Why? 10