Here is the puzzle as it appeared in the AARP Magazine:

  1     +     2     +     3     =     X  
  4     +     5     +     6     =     N  
  7     +     8     +     9     =     ?  


Here are some additional clues from Mr. P:

  1     +     4     +     7     =     E  
  2     +     5     +     8     =     N  
  3     +     6     +     9     =     N  
Solution:


The answer is the letter R.   It is the last letter of the sum (TWENTY-FOUR).

1 + 2 + 3 = SIX
4 + 5 + 6 = FIFTEEN
7 + 8 + 9 = TWENTY-FOUR

In Mr. P's clues:
1 + 4 + 7 = TWELVE
2 + 5 + 8 = FIFTEEN
3 + 6 + 9 = EIGHTEEN

Correctly solved by:

1. Davit Banana Istanbul, Turkey
2. Colin (Yowie) Bowey Beechworth, Victoria, Australia
3. K. Sengupta Calcutta, India
4. Kamal Lohia Holy Angel School,
Hisar, Haryana, India
5. Kelly Stubblefield Mobile, Alabama, USA
6. Ivy Joseph Pune, Maharashtra, India
7. Austin Hale Grand Junction, Colorado, USA