The numbers increase by 2 each step.<br><br>
&bull; 9 to 11 is +2. 11 to 13 is +2.<br>
&bull; Going backwards from 9: 9 &minus; 2 = 7.<br>
&bull; Going forward from 13: 13 + 2 = 15.<br>
&bull; Check: 7, 9, 11, 13, 15 &rarr; each step is +2. &#10003;<br><br>
Final Answer: 7 and 15