Answer
{81}
Work Step by Step
For all positive numbers $a$, where $a\ne1$, and all positive numbers $x$, we know that $y=log_{a}x$ means the same as $x=a^{y}$.
Therefore, $log_{x}9=\frac{1}{2}$ means the same as $x^{\frac{1}{2}}=9$.
We know that $x=81$, because $81^{\frac{1}{2}}=\sqrt 81=9$.