Good day,
Due to my academic background in Mathematics and vast experience of solving problems for clients around the world, I am the right person for this job.
Allow me to make the cover letter simple and short.
You can choose any problem from that textbook "Discrete Mathematics and Its Applications, 7/e
Kenneth Rosen,ISBN: 0073383090,Copyright year: 2012" that you find difficult, and I can provide you with a solution and an explanation over Skype at your convenience, FREE OF COST, as a sample.
If you do not like this proposal but would like to offer something else, then I am all ears. Do not hesitate to let me know what you think.
Best wishes,
Ami Ostad