1. The problem is to simplify the expression $3,21$. 2. It appears there might be a typo or formatting issue since $3,21$ is not a standard mathematical expression. Assuming you meant the number $3.21$ or the expression $3 \times 21$. 3. If it is the number $3.21$, it is already simplified. 4. If it is the multiplication $3 \times 21$, use the multiplication rule: $$3 \times 21 = 63$$ 5. Therefore, the simplified result is $63$ if multiplication is intended.