On the net, there are a several of places that immediately come to mind.
For specific questions that are too tough to handle, try right here on physicsforums. There are many useful websites suggested in the sticky thread at the top of this homework topic.
I recommend two from that list which I have also found helpful.
To learn more about functions and maths, try Math Forum. You can find topics and problems for every level of primary school maths.
Another very handy place for mathematics is MathWorld. They cover a much broader array of topics. I recommend bookmarking this site as a reference for most topics of math you are likely to encounter.