To find the area of the wall not covered by the mural, we subtract the area of the mural from the total area of the wall. The wall's area is 10 ft x 9 ft = 90 sq ft, and the mural's area is 9 ft x 8 ft = 72 sq ft. Therefore, the area not covered by the mural is 90 sq ft - 72 sq ft = 18 sq ft.