Assume that relative maximum and minimum values are absolute maximum and minimum values.
A concert promoter produces two kinds of souvenir shirt; one kind sells for $$ 18,$ and the other for $$ 25 . The total revenue from the sale of $x$ thousand shirts at $$ 18$ each and $y$ thousand at $$ 25 each is given by
$$R(x, y)=18 x+25 y$$
The company determines that the total cost, in thousands of dollars, of producing $x$ thousand of the $$ 18 shirt and $y$ thousand of the $$ 25 shirt is given by $C(x, y)=4 x^{2}-6 x y+3 y^{2}+20 x+19 y-12$
How many of each type of shirt must be produced and sold in order to maximize profit?