Final answer:
The Distributive Property justifies that if 3(x - 5) = 21, then 3x - 15 = 21, as it allows multiplication across the addition or subtraction within a parenthesis.
Step-by-step explanation:
The property that justifies the conclusion If 3(x - 5) = 21, then 3x - 15 = 21 is called the Distributive Property.
This property states that multiplying a number by a sum is the same as doing each multiplication separately.
Therefore, when you distribute 3 over (x - 5), you multiply 3 by x and 3 by -5 separately, which results in 3x - 15.