a)
√121 - √100 + √81 = 11 - 10 + 9 = 10
b)
(√16 + √9)² = (4 + 3)² = 7² = 49
c)
√49 - √36 * √25 = 7 - 6 * 5 = 7 - 30 = -23
d)
√(81√16) = √(81 * 4) = √81 * √4 = 9 * 2 = 18
