which statements describe the procedure Dave used in this step and identifies the property that justifies the procedure?

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  A

Step-by-step explanation:

The second equation differs from the first in that -9 is a term added to both sides of the equation. The addition property of equality tells you that doing this will not change the value of the variable.

The reason (not justification) for doing this is that the sum of 9 and its additive inverse is 0, the additive identity element. This means the equation can be simplified to 3x = 12, which is the point of making that step (subtracting 9).