The next number in the sequence is always found by subtracting the number before the previous number from the previous number. So you calculate the number in position 3 by subtracting the number in position 1 from the number in position 2.

Watch out, subtracting a negative number is the same as adding the number.

117 - 9 = 108

108 - 117 = -9

-9 - 108 = -117

-117 - -9 = -108