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

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

11 - 110 = -99

110 - -99 = 209

-99 - 209 = -308

209 - -308 = 517

-308 - 517 = -825

517 - -825 = 1342