As L can be anything from A to Z , there are 26 combinations for that and as repetition is allowed, for second and third letters, we again have 26 combinations available and thus 26×26×26=17576 combinations for letters.
You have to understand the problem clearly before trying to answer the question. Only thing that needs to be done to answer the question is adding 5 with f(x). This will yield the required result.
