My math is not on solid ground.

I would say as a student and tutor, that it is fine to use.

Solid can be used slang in sentences like this. It's not proper English but younger people could probably understand. It's about imagery, as the listener can understand that something that is not solid is not perfect. Another example; "This party is lit!", implies the party is "on fire!" in a non-literal sense meaning it was a very active party.

I depends on what you are trying to say. I would think that this is an awkward way to say it. I would say "I am not comfortable with my math skills" or "I am not sure about math."

Solid does not mean familiar in either the dictionary or the thesaurus. http://www.learnersdictionary.com/definition/solid http://www.thesaurus.com/browse/solid?s=t