|A) U||B) G|
|C) H||D) I|
It means that the first half of the alphabet is interchanged with second half of the alphabet. After making changes, nth letter from the left will be (n + 13)th letter of original series, ie English alphabet; If n is less than 14. But if n is more than 13, nth letter from left will be (n - 13)th letter from left.
In the given equation, we have to find out (13 - 7 =) 6th letter from right => (26 - 6 + 1 =) 21st letter from the left. Hence, required letter's position is more than 13; therefore the required letter is (21 - 13 =) 8th letter from left of original series, ie H.