Khan academy: a helpful, free online resource for math and more!
Is your child struggling in math? Do they need extra help to understand a concept?
The first place they should go for extra help is to their teacher.
If they still need more help, try khanacademy.org.
For example, if your student is struggling to understand slope, search for slope and find multiple supporting videos and practice problems; see slope example in this screen shot from KhanAcademy.org.