Synchronous orbit around Mars

Mars has mass $6.42\cdot 10^{23}\mathrm{kg}$ and rotates about its axis in $88642\mathrm{s}$. Compute the radius of the equatorial circular orbit of a satellite that always stays above the same point of the Martian surface ($G=6.67\cdot 10^{-11}\mathrm{N}\cdot {\mathrm{m}}^2/k{\mathrm{g}}^2$).