Elimination Method: https://www.khanacademy.org/math/algebra-basics/core-algebra-systems/core-algebra-systems-tutorial/v/solving-systems-of-equations-by-elimination
Substitution Method: https://www.khanacademy.org/math/cc-eighth-grade-math/cc-8th-systems-topic/cc-8th-systems-with-substitution/v/the-substitution-methodhttps://www.khanacademy.org/math/cc-eighth-grade-math/cc-8th-systems-topic/cc-8th-systems-with-substitution/v/the-substitution-method
Graphing Method: https://www.khanacademy.org/math/cc-eighth-grade-math/cc-8th-systems-topic/cc-8th-systems-graphically/v/graphings-systems-of-equations
Side Note: Not only does Kahn Academy allow for watching videos to help learn these skills but they also have practice problems as well. Feel free to look around the site and use what is helpful for you. We have used this for class quite a bit and I feel that it can be very helpful for the students.