Find the missing number?

Answer: Option A

In each star, the number in the centre equals the difference between the sum of the numbers in the upwards triangle and the sum of the numbers in the downward triangle. So,

For First Star,
(4 + 9 + 2 ) - (3 + 1 + 6) = 15.

Second Star,
(6 + 8 + 5) - (7 + 3 + 1) = 8.

Third Star,
( 6 + 9 + 1) - (6 + 1 + 2) = X

X = 7.