For Q17b, note that the area (Q+R) is equal to 1/2 of the square. Since area (P+S) + area (Q+R) = area of square, the area (P+S) must be 1/2 of the square as well. Since the area of P is 1/9 of the square, area of S = 1/2 - 1/9 = 7/18.