Chapter 6 - Systems of Equations and Inequalities - 6-3 Solving Systems Using Elimination - Lesson Check - Page 381: 5

Using the Addition Property of Equality, which states that when a=b then a+c = b+c, we can add values to one side of an equation and do the same to the other side of the equation. This is because the total balance of the equation will stay the same. Using this theory allows for us to add equations as long as we do the same to both sides.

See above.