Saturn's moon Iapetus has a mean density of about 109 kgm-3. It’s equatorial ridge bisects its entire dark hemisphere an

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Saturn's moon Iapetus has a mean density of about 109
kgm-3. It’s equatorial ridge bisects its entire dark
hemisphere and rises to a height of about 19.6 km. It could
have formed from the collapse of a ring formed from the tidal
disruption of a moon it once had. Another idea is that is that
volcanic forces formed the ridge.
If the ridge is entirely H2O ice with a density of
91.6 kgm-3 estimate how deep below the surface of
Iapetus the roots of this ice mountain ridge would go.
Assume that the below and surrounding the pure ice the density
is the mean density of Iapetus. (Answer with 3 sig. figs. in
km)