Putting A = 1, B = 2, ..., Z = 26, we get the given matrix as
Clearly, 2 x (2nd column - 1st column) = 3rd column - 2nd column.
Let the missing letter be the nth letter of the alphabet.
Then, 2 x (10 - 5) = n - 10 or n - 10 = 10 or n = 20.
So, the missing letter is the 20th letter, which is T.