The sum of 9 and three times a number is 21. what is the number?
equation:

n =

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 :)