I doubt that the length of the wall is  11 1/4 inches.  You probably meant 11 1/4 feet.
Subtract 8 7/8 feet from 11 1/4 feet. Â In other words, subtract
8 7/8 ft from 10 10/8 ft (after borrowing). Â 10 10/8 ft - 8 7/8 ft = 2 3/8 ft.
Divide this in half. Â On each side of the picture will be 1 3/16 ft of wall.