Find the missing number?

Solution (By Examveda Team)

Split the whole diagram in half, horizontally, to give two groups of 5 octagons. In each group, the central number in the top row equals the sum of the surrounding 4 numbers.

13 = 3 + 2 + 7 + 1

So,
14 = 6 + 3 + x + 1
x = 14 - 10 = 4.