1. **State the problem:** Calculate the sum of 6 and 3. 2. **Formula used:** Addition of two numbers is given by $a + b$. 3. **Apply the formula:** Here, $a = 6$ and $b = 3$, so we calculate: $$6 + 3$$ 4. **Perform the addition:** $$6 + 3 = 9$$ 5. **Final answer:** The sum of 6 and 3 is 9.