Chapter 6 solving systems

Elimination:

http://www.purplemath.com/modules/systlin5.htm
http://www.mathplanet.com/education/algebra-1/systems-of-linear-equations-and-inequalities/the-elimination-method-for-solving-linear-systems

Graphing:

http://www.regentsprep.org/regents/math/algebra/AE3/GrSys.htm
http://www.virtualnerd.com/tutorials/?id=Alg1_11_1_1

Substitution:

http://www.regentsprep.org/regents/math/algebra/AE3/AlgSys.htm
http://coolmath.com/algebra/12-2x2-systems-of-equations/02-solving-by-substitution-01.htm

Also

https://www.khanacademy.org/math/algebra/systems-of-eq-and-ineq