1. **State the problem:** Simplify the expression $3x(-1)x^8$. 2. **Recall the rules:** When multiplying terms with the same base, add the exponents: $x^a \cdot x^b = x^{a+b}$. 3. **Simplify step-by-step:** - Multiply the constants: $3 \times (-1) = -3$. - Multiply the $x$ terms: $x^1 \times x^8 = x^{1+8} = x^9$. 4. **Combine the results:** The simplified expression is $-3x^9$. 5. **Final answer:** $$-3x^9$$