The question, as stated, cannot be answered sensibly. A kilometre is a measure of distance, with dimensions [L]. An hour is a measure of time, with dimensions [T]. The two measure different things and basic dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information.
As a simple mental exercise, consider how long it would take you to walk 12 km compared with the time it would take you to drive that distance.
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Assuming "3 12" is meant to be 3 1/2 then it will be 28 km.
if you are not moving it is ∞ hours. If on the other hand you are moving with ∞ Km/h then that would be 0 hours.
Answer: 12 km = 7.45645 miles.
12 km = 39370 feet (approx).
1 km = 1000 metres so 12 km = 12*1000 = 12000 metres. Simple!