1. **State the problem:** Calculate the value of $6 - (-3)$. 2. **Recall the rule:** Subtracting a negative number is the same as adding its positive counterpart. In other words, $a - (-b) = a + b$. 3. **Apply the rule:** Replace $-(-3)$ with $+3$, so the expression becomes $6 + 3$. 4. **Perform the addition:** $6 + 3 = 9$. 5. **Final answer:** $6 - (-3) = 9$.