Hi, I am having a hard time with these three parts of this problem. I have done several of these problems so far and have gotten them all correct but this one is showing my work is wrong? What am I missing?

Consider the function f (x) = x + 15x 3 (c) Give the intervals of increase and decrease of f (I). Note: Use the letter U for union. To enter OO, type infinity with a lower case i. If the function is never increasing or decreasing, enter NA in the associated response area. increasing (0, infinity) decreasing: (-infinity, -1000) U (-1000, 0) 6 (d) Give the local maximum and minimum values of f (I). Enter your answers in increasing order of the r-value. If there are less than two local extrema, enter NA in the remaining response areas and the corresponding drop-down menu. Include a multiplication sign between symbols. For example, a - IT. f( -1000 500 OGF is a local maxi... na na P NA (e) Give the intervals of concavity of f (I). Note: Use the letter U for union. To enter 00, type infinity with a lower case i. If the function is never concave upward or concave downward, enter NA in the associated response area. concave upward: concave downward: na