Answer
$x^{5a}$
Work Step by Step
Based on the quotient rule for exponents, we know that $\frac{a^{m}}{{a}^{n}}=a^{m-n}, a\ne0$ (where $m$ and $n$ are positive integers and $a$ is a real number).
Therefore, $\frac{x^{9a}}{x^{4a}}=x^{9a-4a}=x^{5a}$.