This question is tricky. I take it that you are supposed to assume that all of the other elements in T are generated by 2 and 9. To begin, lets list out several members of T: 2, 9, 6 (2x2 +2), 10 (2x2 +6), 13 (2x2 +9), 14 (2x2 +6 or 2x6 +2), ...21 (2x6 +9) and then keep adding 4 (2x2) and you'll get 25, 29, 33. So, we can see that 33 is in T.
I actually started with finding which element isn't in T, and it's 15. We know 6 is in there, and 14 is an easy step from there. But, notice that 15 is odd, which means that it is the sum of an even and odd number. The fist odd is nine, and we find another until 13. But, 15 is not equal to 2x2 +9, which means that you have to move from 2 to the next element in T other than 9, but 2x6 +9 is also not 15.
Like I said, this is tricky, but I hope that it makes sense. Let me know if you need me to clarify or rewrite anypart.