1 8 7 3 ( ) 17 1 1 15 16 9

Actually, the distance is like this:

8- 1=7 7-8=- 1 3-7=-4 () 1 1- 17=-6 15- 1 1=4 16- 15= 1 9- 16=-7

You will find a rule:

7 - 1 -4 ()-6 4 1 -7

The difference between the two sides is one symbol.

Then () is: 3+6=9.