Find the absolute minimum and absolute maximum of f(x, y) = 4 - 8x + 9y on the closed triangular region with vertices (0, 0), (9, 0) and (9, 10). List the minimum/maximum values as well as the point(s) at which they occur. If a min or max occurs at multiple points separate the points with commas. Minimum value: Occurs at Maximum value: Occurs at