Answer
$4.22 \times 10^7 \ m$
Work Step by Step
We solve the equation for the period of the orbit for r to find:
$r = (\frac{GM_ET^2}{4\pi^2})^{1/3} = (\frac{6.67\times10^{-11}\times5.97 \times 10^{24}(8.64 \times 10^4)^2}{4\pi^2})^{1/3} = 4.22 \times 10^7 \ m$
