.
You will be surprised, but even among mathematicians there is no one single canonical definition of what are "natural numbers".
The most widely accepted understanding is that they are integer numbers {1, 2, 3, . . . }, starting from 1.
But there are influential mathematical schools and countries (France, for example), where they define natural numbers
including 0 (zero) and starting from it.
You can read about it from this Wikipedia article https://en.wikipedia.org/wiki/Natural_number.
In US and in US schools, the natural numbers are integer starting from 1.
By accepting this definition, the answer to your question is this set {3, 5, 7, 9}.
These numbers correspond to values x= 1, 2, 3 and 4 in the formula y = 2x + 1.
Solved, explained and completed.
