By the end of this section I can...
-solve a system of linear equations by substitution
Khan Academy Videos and Interactive Practice:
https://www.khanacademy.org/math/algebra-basics/core-algebra-systems/core-algebra-systems-tutorial/v/solving-linear-systems-by-substitution
https://www.khanacademy.org/math/algebra/systems-of-linear-equations/solving-systems-of-equations-with-substitution/v/solving-systems-with-substitution
https://www.khanacademy.org/math/algebra/systems-of-linear-equations/solving-systems-of-equations-with-substitution/v/practice-using-substitution-for-systems
https://www.khanacademy.org/math/algebra/systems-of-linear-equations/solving-systems-of-equations-with-substitution/e/systems_of_equations_with_substitution
Other Resources:
http://www.purplemath.com/modules/systlin4.htm
http://www.mathplanet.com/education/algebra-1/systems-of-linear-equations-and-inequalities/the-substitution-method-for-solving-linear-systems
https://braingenie.ck12.org/skills/107882
https://braingenie.ck12.org/skills/105516
https://braingenie.ck12.org/skills/105517