For metric conversion, you can remember this acronym for help:
King Henry died unusually drinking chocolate milk. Which stand for:
Kilo - unit * 1000
Hecto - unit * 100
Deca - unit * 10
Unit - unit * 1
Deci - unit *
Centi - unit *
Milli - unit *
Kilometers and centimeters are five places apart apart, so you move the decimal point in 664.2 to the right five times, which means 664.2 km = 66420000 cm.
To avoid confusion on which direction to move the decimal point, imagine two shapes on each end of a scale. On each end, there is one large shape and one small shape. There has to be one of each on either side for it to balance. For this problem, a kilometer is a larger unit than a centimeter, so this means that the blank space needs to have a number greater than 664.2, or else the scale won't balance. Hope this helped.