Solution:The pattern followed here is :(1st number - 2)² = 2nd number
(11 : 81) → (11 - 2)² ⇒ 9² = 81
(22 : 400) → (22 - 2)² ⇒ 20² = 400
Similarly,
(x : 225) → (x - 2)² = 225
⇒ x² - 4x + 4 = 225
⇒ x² - 4x - 221 = 0
⇒ x² + 13x - 17x - 221 = 0
⇒ x(x + 13) - 17(x + 13) = 0
⇒ (x + 13) (x - 17) = 0
x = - 13 or x = 17
Hence, "17" is the correct answer.