To find the unknown number, we can set up an equation based on the information you provided.

Let the unknown number be represented by $x$.

The sum of 6, 8, and $x$ is 21, so we can write:

$6 + 8 + x = 21$

Now, simplify the left side:

$14 + x = 21$

Next, subtract 14 from both sides to isolate $x$:

$x = 21 - 14$

$x = 7$

So, the unknown number is $7$.

Would you like to go over any details or need further assistance?

Here are some related questions:

1. What is the method for solving a simple equation like this?
2. How can you check if the solution is correct?
3. What happens if the sum were a different number, say 25?
4. Can you solve this problem using subtraction instead of addition?
5. How would you represent this kind of problem algebraically if there were more than one unknown number?

Tip: When solving equations, always perform the same operation to both sides to maintain balance!