First you divide both sides of the equation by 3. This leaves you with 9-2=7 (notice I rewrote 9+(-2) to 9-2, this is because they mean the same thing one is just easier to look at). Then you add 2 to both sides. This cancels the 2 on the left side and makes the rights side a nine. Since both sides leave you with nine this proves are equal, making it a true equation.
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