ok so for this you need to know how to convert measurements.
12.5km needs to be in cm. So you need to know what unit conversion is going to get me cm and sometimes its more then 1 step.
So one way is we could use these conversions km to m and then m to cm
So turn 12.5km into a fraction over 1 (12.5/1km)
and times it by 1000m over 1km you should get 12500m.
Next take 12500m over 1 and multiply it by 100cm over 1m as a fraction so then your anwser should be 1,250,000cm