Work Step by Step
To find the answer you could find the GCF and factor the expression, but in a standardized test you should try to solve each question as quickly as possible. Use mental math to find the answer more quickly. You know that $ax(3x^2)$ must equal $9x^3$, so $a\times3=9$. a must be 3. You also know that $ax(5)$ must be 15. In this equation, a must also be 3. This validates the result.