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.
14 - 28 = -14
28 - -14 = 42
-14 - 42 = -56
42 - -56 = 98
-56 - 98 = -154