Here are YouTube channels and other video sites that cover middle school math curriculum. These are great to search through and find the topic you need for reinforcement. 

Virtual Nerd - One of my favorites and offers super short, to the point videos.

Math with Mr. J - Math with Mr. J is a math education channel that offers instructional math videos to anyone looking for a little extra help with math!

The Magic of Math - These videos address foundational Math topics for grades 6 - Algebra 1 per Common Core Math Standards.

Mrs. D Math - Free Math tutoring Mvideos for 6th, 7th, 8th & Algebra students created by a teacher!

Mrs. V's Room - These videos address 7th grade common core standards.

Simplify the Middle - This channel is dedicated to math educational videos.

Mrs. Casias Math - Mrs. Casias is an 8th grade math teacher from Texas who makes videos over 7th grade math, 8th grade math Pre-Algebra and Algebra 1 Lessons!

​Khan Academy - scroll down to find the grade level and then click on a topic