We are solving for n. The first thing to do when solving for any variable is to add like terms.
10n+12-9-7n+4n=31
10n, -7n, and 4n are like terms because they have the same variable(n). 12 and -9 are like terms because there are both constants.
10n+12-9-7n+4n=31
7n+3=31
-3=-3
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7n=28
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7 7
n=4
If you want to check your answer, you simply plug the variable back into the displayed equation. Have a good day!