A map of a small regional park is a rectangular grid, bounded by the lines x = 0, x = 4, y = 0, and y = 3, where units are in miles. It is found that the elevation above sea level at each point (x, y) in the park is given by E(x, y) = 90(2x + y²) feet Find the average elevation in the park. (Remember, 1 mi = 5,280 feet.)