Let’s break this into two parts — numbers and letters.
Number Series:
3 → 6 ( +3 )
6 → 11 ( +5 )
11 → 18 ( +7 )
18 → ? ( +9 ) ⇒ 27
Letter Series:
F (6), G (7), L (12), L (12), ?
Now look at pattern:
F (6) → G (7) → L (12)
Then again L (12), and next should be +3 ⇒ P (16)
Final Answer = 27P