A linear function f (x, y) = ax + by + c has no critical points. Therefore, the global minimum and maximum values of f (x, 3;) on a closed and bounded domain must occur on the boundary of the domain. Furthermore, it can be easily seen that if the domain is a polygon, [hen the global minimum and maximum values of f must occur at a vertex of the polygon. Find the global minimum and maximum values of f (x, y) = 2x - 6y 6 on the domain where |x| + IyI g 9. (Give your answers as a whole numbers.)