Answer
$6.9\ Mm$
Work Step by Step
We know that kinetic energy plus potential energy is constant. Using this fact and solving for r, we find:
$r = (\frac{1}{R_E}-\frac{v_0^2}{2GM_E})^{-1}=(\frac{1}{6.37 \times 10^6}-\frac{3100^2}{2(6.67\times10^{-11}\times5.97 \times10^{24})})^{-1} = 6.9 \ Mm$
