Answer
$x=2$
Work Step by Step
We know that if $b\gt0$ and $b\ne1$, then $y=log_{b}x$ is equivalent to $x=b^{y}$ (where $x\gt0$ and $y$ is a real number).
Therefore, $log_{x}8=3$ is equivalent to $x^{3}=8$. Since $x\gt0$, we know that $x=2$, as $2^{3}=8$.