Find the absolute maximum and minimum values of f on the set D.

f (x, y ) = 4xy^{2} − x^{2}y^{2} − xy^{3} ; D is the closed triangular region in the xy -plane with vertices (0, 0), (0, 6), and (6, 0).

f (x, y ) = 4xy^{2} − x^{2}y^{2} − xy^{3} ; D is the closed triangular region in the xy -plane with vertices (0, 0), (0, 6), and (6, 0).