69 * 31 = 2,139 so the area of the whole complex as a rectangle is 2,139m but we don't want the whole rectangle since there is a square cut out. so just subtract the area of the square with the area of the whole rectangle. 12 * 23 = 276 so the little cut out square is 276m. so do 2, 139m - 276m which is 1,863m.
