All the material published/posted on MathCity.org is send by different peoples to help the other people for the promotion of mathematics. These are usually open educational resources (OER).
OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use or re-purposing by others. Open educational resources include full courses, course materials, modules, textbooks, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge. (see more at wikipedia)
If you think that there is a material posted on MathCity.org, which is unauthorized copying of copyrighted material or which is an act of copyright infringement, then please contact with “admin [at] mathcity [dot] org with details.