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.
10 - 120 = -110
120 - -110 = 230
-110 - 230 = -340
230 - -340 = 570
-340 - 570 = -910
570 - -910 = 1480