Dear Sir/Madam,
Thanks to my aptitude for Mathematics, my extensive knowledge of almost all topics at Undergrad level Mathematics and most topics at Postgrad level Mathematics backed up by my ample experience, I am well placed to help you out with your problems in Combinatorics, including explaining simple topics or concepts like the Pigeonhole principle, inclusion-exclusion principle, generating functions, and other topics generally covered at the (presumably) Undergrad level course that you are preparing for.
Thanks and regards,
Akram Hussein