Which of the following letter will replace the question mark (?) and complete the letter series? J, D, X, R, L, ?

F D  E  G

F

J, D, X, R, L, ? The logic followed here is : The difference between consecutive terms is 6, such that: J → (-6) → D → (- 6) → X → (-6) → R → (-6) → L  → (-6) → F