This question is all about theory and estimation of cost in a managerial economics subject.

1. Given the production function Q = 21X + 9X^2 - X^3, where Q = Output, and X = Input, at what value of X does Stage II of the production function begin?
2. Given the production function Q = 21X + 9X^2 - X^3, where Q = Output, and X = Input, at what value of X does Stage III of the production function begin?
3. Given the production function Q = 21X + 9X^2 - X^3, where Q = Output, and X = Input, at what value of X does diminishing returns set in?