This "1676401km/h" can't be right. A geostationary satellite orbits at 3.07 km/s, or 11,052 km/h.
Okay, the problem is done like this:
2*pi*D = speed * 24 h, if you plug your 1676401km/h:
2pi*D = 1676401*24 ---> D = 6,403,365 km
But again you should make sure what the speed is in the statement. Best!