Here is what I'd do.....

(i) There are 26 possible letters, each set of initials would have 3 letters in it, order doesnt matter, i.e. ABC does not equal ACB. So type in 26P3 on your calculator (P=permutation).

(ii) This time each set of initials can have 2 or 3 values, so calculate 26P3 + 26P2.

(iii) 26P4 + 26P3 + 26P2.

(iv) x = solution for (ii) - 20,000

Hope this helps...