10n+12-9-7n+4n=31, does anyone know the right answer

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

7n=28
---  
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!

Just look outside and then you’ll see where the awnser is