For this case we have: 
By definition, the Addition Property of Equality establishes that an equality is not altered as long as the same number is added on both sides of it. That is to say: 
 can be rewritten as:
 can be rewritten as: 

If we have: 

We can add 6 to both sides of the equation, obtaining the following: 

Proving that the statement is true. 
Answer: 
Addition Property of Equality