1. **State the problem:** Simplify the expression $4X + 5X$. 2. **Formula and rules:** When adding like terms, add their coefficients and keep the variable the same. 3. **Intermediate work:** $$4X + 5X = (4 + 5)X$$ 4. **Simplify:** $$= 9X$$ 5. **Explanation:** Since both terms have the variable $X$, we add the coefficients $4$ and $5$ to get $9$, then multiply by $X$. **Final answer:** $9X$