To solve equations like this we need to do inverse operations.
First we need to build an equation: 9+3x=21
We pass the nine to the other side, subtracting the 9 to 21, that leaves us with 3x=12
After that we just divide 12 by 3, wich gives us x=4, or, as it says in the question: n=4
Now let's double check:
N=4, since 3*4=12 and 12+9=21
Hope it is helpful :)
