Use a graph or level curves or both to find the local maximum and minimum values and saddle points of the function. Then use calculus to find these values precisely. (Enter your answers as a comma-separated list. If an answer does not exist, enter DNE.) f( x , y ) = 9( x - y)e -x2 - x2 local maximum value(s) X local minimum value(s) X saddle point(s) ( x, y, f ) = DNE