Answer: From here, we can simplify further if needed, but the Subtraction Property of Equality justifies the given step of 7x - 21.
Explanation:
The step provided in the question is 7x - 21. To justify this step, we need to determine which property of equality was used.
In this case, the appropriate property to justify the step is the Subtraction Property of Equality. This property states that if we subtract the same quantity from both sides of an equation, the equation remains true.
In the given step, we can see that 7 is subtracted from both sides of the equation. By applying the Subtraction Property of Equality, we can conclude that the equation remains true:
7x - 7 - 7 = 0 - 7
7x - 14 = -7