1. **State the problem:** Simplify the expression $X + 12 - 15$. 2. **Understand the operation:** Addition and subtraction are performed from left to right. 3. **Combine like terms:** $$X + 12 - 15 = X + (12 - 15)$$ 4. **Simplify the constants:** $$12 - 15 = \cancel{12} - \cancel{15} = -3$$ 5. **Write the simplified expression:** $$X - 3$$ **Final answer:** $X - 3$