Usually you will know whether the sequence starts at 0 or 1 and continues upward by 1. Usually sequences are represented by the letter n.
If the equation you are dealing with is 2*n+3 and you need to get the first 4 numbers from this sequence, and assuming you need to start from 0, the answer is as follows:
2*0+3, 2*1+3, 2*2+3, 2*3+3 = 3, 5, 7, 9 = the first 4 numbers of this sequence.