The first thing is that an equation has to have an equals sign somewhere. If it doesn't it isn't an equation. But it's probably an expression. So, since you haven't used the word "equals" or "is", we probably have an expression so we write it as we see the words: 21+3n-n-11. This expression simplifies because we can combine "like" terms, that is, pure numbers (21 and 11 in this case) making sure we observe the signs in front of the numbers; and variables (sometimes called unknowns). We have 21-11=10.
We also have 3n-n=2n. The expression then simplifies to 10+2n.