you have 4 walls, 2 with length 12 feet, 2 with length 15 feet and each of them has a height of 12 feet so you sum all the lengths and then multiply it by the height to get the total area
total length=2*(12+15) =2*(27) =54 total area=total length*ceiling height =54*12 =648 feet^2 to cover all walls with wallpaper
