1. **State the problem:** Simplify the expression $x - (-3)$. 2. **Recall the rule:** Subtracting a negative number is the same as adding its positive counterpart. Formally, $a - (-b) = a + b$. 3. **Apply the rule:** Replace $-(-3)$ with $+3$. $$x - (-3) = x + 3$$ 4. **Final answer:** The simplified expression is $x + 3$.