Why we do the same to both sides of equations
https://www.khanacademy.org/math/algebra/one-variable-linear-equations/why-of-algebra/v/why-we-do-the-same-thing-to-both-sides-simple-equations
Representing a relationship with a simple equation
https://www.khanacademy.org/math/algebra/one-variable-linear-equations/why-of-algebra/v/representing-a-relationship-with-a-simple-equation
How to solve one step equations
https://www.khanacademy.org/math/algebra/one-variable-linear-equations/why-of-algebra/v/adding-and-subtracting-the-same-thing-from-both-sides
Equations of Proportional relationships
https://www.khanacademy.org/math/cc-seventh-grade-math/cc-7th-ratio-proportion/cc-7th-equations-of-proportional-relationships/v/equations-of-proportional-relationships
EXAMPLE -- Equations of Proportional relationships
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
Interpreting graphs of proportional relationships
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