The marketing department of a company has determined that if it spends x thousands of dollars on radio advertisements and y thousand dollars on newspaper advertisements, the company's revenue (in thousands of dollars) will be R(x, y) = - 0.05x^2 - 75y^2 + 5x + 6y + 3xy. How much money should this company spend on radio advertisements and newspaper advertisements to maximize its revenue?
Radio: $
Newspaper: $