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.
8 - 56 = -48
56 - -48 = 104
-48 - 104 = -152
104 - -152 = 256
-152 - 256 = -408
256 - -408 = 664