https://www.khanacademy.org/math/cc-seventh-grade-math/cc-7th-ratio-proportion/cc-7th-equations-of-proportional-relationships/v/equations-of-proportional-relationships
VIDEO #2 -- Writing proportional equations
https://www.khanacademy.org/math/cc-seventh-grade-math/cc-7th-ratio-proportion/cc-7th-equations-of-proportional-relationships/v/constructing-an-equation-for-a-proportional-relationship
VIDEO #3 -- Interpreting proportional graphs
https://www.khanacademy.org/math/cc-seventh-grade-math/cc-7th-ratio-proportion/cc-7th-equations-of-proportional-relationships/v/examples-interpreting-graphs-of-proportional-relationships