how do I convert 2.56m/s^2 to km/s^2
you can always multiply by 1 without changing anything.
Since 1km = 1000m,
1km/100m = 1000m/1km = 1
2.56m/s^2 * 1km/1000m = 2.56/1000 km/s^2
This method allows you to convert between units without wondering whether to multiply or divide. Just use the value of 1 which cancels the unwanted units.