Complete the series: _______, 150, 183, 219, 258

1. Calculate the differences between the given consecutive numbers: 183 - 150 = 33; 219 - 183 = 36; 258 - 219 = 39.
2. Observe the pattern in the differences: +33, +36, +39. The differences are increasing by 3.
3. Working backward, the difference before +33 must be +30.
4. Subtract 30 from 150 to find the missing first term: 150 - 30 = 120.