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The correct answer is 1.
Solution:
Go clockwise around the star. Add the number on the outermost points of the star to the following number on a inner point of the star to get the number on the next outermost point.
2 + 3 = 5
5 + 1 = 6
6 + 7 = 13
13 + 2 = 15
15 + (-13) = 2