1. **State the problem:** Simplify the expression $120x^8 + 240x^8 + 360x^?$ where the exponent of the last term is unknown. 2. **Identify the terms:** The first two terms are like terms because they have the same variable and exponent $x^8$. 3. **Add the like terms:** $$120x^8 + 240x^8 = (120 + 240)x^8 = 360x^8$$ 4. **Analyze the last term:** The last term is $360x^?$ which is incomplete because the exponent is missing. 5. **Conclusion:** Without the exponent of the last term, we cannot combine it with the first two terms. The simplified expression for the known terms is: $$360x^8 + 360x^?$$ If the exponent of the last term is also 8, then: $$360x^8 + 360x^8 = 720x^8$$ Otherwise, the expression remains as above. **Final answer:** $$360x^8 + 360x^?$$