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.
12 - 144 = -132
144 - -132 = 276
-132 - 276 = -408
276 - -408 = 684
-408 - 684 = -1092
684 - -1092 = 1776