Let's analyze each step:
1. 7x
This step represents the equation without any changes.
2. 7 - (-7)
In this step, we have subtracted -7 from both sides of the equation. This is done using the Subtraction Property of Equality, which states that if you subtract the same number from both sides of an equation, the equality still holds true.
3. 7x = -21
Here, the result of the subtraction in the previous step simplifies to -21. This step represents the equation after simplification.
4. x = -3
In this step, we have divided both sides of the equation by 7. This is done using the Division Property of Equality, which states that if you divide both sides of an equation by the same nonzero number, the equality still holds true.
Therefore, the appropriate property to justify step 2 is the Subtraction Property of Equality, and the appropriate property to justify step 4 is the Division Property of Equality.