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.

5 - 10 = -5

10 - -5 = 15

-5 - 15 = -20

15 - -20 = 35

-20 - 35 = -55