Part A: In the x-intercepts, it's showing how he earns no profit when he sells that amount of goods. The maximum shows that that's the most profit he ears.
Part B: Increasing interval - (0,3) and Decreasing interval - (3,6]. The intervals show that his profits increase if he stays under 3 and above 0 good, and his profits decrease when he sells above 3 goods.
Part C: I think the average rate of change is 40x because when x-value is 3 then y-value is 120, and 120/3 is 40. This shows how his profits correlate with the number of goods he sells.
Hope this helps :)