6 hours is a quarter of a day. We go through 2π radians in one day (full revolution) so in six hours we travel through π/2 rad.
Velocity = 2πr/T (where T is the period of 24 hours)
so our orbital velocity = 2xπx6370000/24x60x60
= 463.2 ms<sup>-1</sup> I hope it's right (I gotta head off) and I hope it helps
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