\(\frac{1}{8} \frac{\mu_0 i}{2 r}=\frac{\mu_0 / r^2}{2\left(r^2+x^2\right)^{3 / 2}}\)
\(\Rightarrow\left(r^2+x^2\right)^{3 / 2}=8 r^3\)
\(\Rightarrow\left(r^2+x^2\right)=\left(8 r^3\right)^{2 / 3}\)
\(\Rightarrow r^2+x^2=4 r^2\)
\(\Rightarrow 3 r^2=x^2\)
\(\Rightarrow x=r \sqrt{3}\)