7.1 Systems of Linear Equations in Two Variables, 8.1 Systems of Linear Equations: Substitution and Elimination, Sullivan Chapter 8, Trigsted Chapter 7 Systems of Equations: Linear System with two variable by Substitution July 19, 2017 admin https://www.youtube.com/watch?v=pvy1r1soht8 https://www.youtube.com/watch?v=kf-o_CcTKH8