A new computer device will be submerged just outside the ship channel fairway. In order to get the most accurate readings the device must be located at a point such that the distance from the bay floor to the device is equal to one-third of the distance between the device and the top of the antenna. The depth of the bay at the selected location is 18.5 feet. The height of the ante will be 21.5 feet above the water at mean tide. Determine the depth at which the computer device is located at mean tide.