Expression 4: "r" Subscript, "y" , Baseline left parenthesis, "N" Subscript, "x" , Baseline , "N" Subscript, "y" , Baseline , "D" Subscript, "x" , Baseline , "D" Subscript, "y" , Baseline , right parenthesis equals "D" Subscript, "y" , Baseline minus 2 "N" Subscript, "y" , Baseline left parenthesis, "N" Subscript, "x" , Baseline "D" Subscript, "x" , Baseline plus "N" Subscript, "y" , Baseline "D" Subscript, "y" , Baseline , right parenthesisryNx,Ny,Dx,Dy=Dy−2NyNxDx+NyDy