i'm gonna try to describe this as best as i can:
to get from 1 to 3, you must add two. therefore making 2 the third and next number in the sequence.
to get from 2 to 6, you must add four, therefore making 4 the fifth and next number in the sequence.
to get from 4 to 9, you must add five, therefore 5 would be the seventh and next number in the sequence.
i hope this helps you out!
