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.
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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).
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