See: 
Mentally &/or Graphically Solve Systems w/Appropriate Vocabulary & Possible Solutions
 Use Linear Combination (the Addition Method)
to Solve Systems of Equations and Inequalities
 Gives instructions and examples.
 Solve Graphically w/Planes &Pencil (Includes Answers)
 Solver (Using Web Page)
 By Linear Combination (Adding Lines)
 By Substitution (Expression Swapping)
