Click the 'edit' button at the top of any section screen. Follow the example of the problems that are already up to make the link. Then save the page and click on the new 'solution' link that you created. That will take you to the problem page where you should write again the problem and then the solution, with a nice footer that links back to the section. Just follow the other problems!
The math code works just like TeX. I suggest looking up symbols and functions in other problems and seeing how it's done there.
Here's a good list of symbols: http://meta.wikimedia.org/wiki/Help:Formula
Here is a bunch of TeX documentation:
Thank You for contributing!