This is simply a question about ratios. This ratio means that for every 1 cm on the map, there are 12 km in actuality. You can set this problem up in fractions:
1 cm 12 cm
-------- = ----------
12 km x
To solve for x, we cross-multiply, for the moment ignoring units. Remember, your answer will be in km.
1*x=12*12
x=144
If x=144, then for 12 cm, there are 144 km.