i think that is a great idea !
and i agree with what people said just post adds in libraries
and maybe in front of math classes this way when we are getting out or coming into class we can see the add
also i'm not sure if profs would do this (i don't see why they wouldn't) , but maybe you could tell first year calc profs that you are offering help and this way the prof could announce that to his classes
btw if you don't mind but why are you offering free help ( not that i'm complaining) ? usually people want money
best of luck !!