Answer :

2 -> 7; but 7 = 2 * 2 + 3;
3 -> 9; but, 9 = 3 *2 + 3;
4 -> 11; but, 11 = 4 * 2 + 3;
5 -> 13; but, 13 = 5 * 2 + 3;

The rule is : ( your number ) * 2 + 3;

The function rule is f( x ) = 2x + 3.