The next number in the series is always found by adding the previous number to the number before it. So you calculate the number at position 3 by adding the number at position 2 to the number at position 1.
28 + 2 = 30
30 + 28 = 58
58 + 30 = 88
88 + 58 = 146
146 + 88 = 234
234 + 146 = 380