if the area of an object is define as A=5xy^2-9yx given x and y are variable how do we find the combination of x and y that will give the maximum or minimum value(if it does have a maximum or minimum point)

∂A/∂x=∂A/∂y=0,let 'delta'=(∂²A /∂x²)(∂²A/∂y²)-∂²A/∂x∂y,if delta>0 and ∂²A /∂x² and ∂²A /∂y² are both less than zero, it is a maximum, if both are greater than zero, it is a minimum

hmm, so what is the coordinate of the point