Your answer would be 2km.
We can write the scale as a ratio of 2cm:5km. Then we need to find out how much we multiply 2 by to get 0.8cm, by dividing 0.8 by 2. This gives us 0.4.
Then, we can multiply 0.4 by 5 on the other side of the ratio, to get the ratio as 0.8:2, so 2km is your answer.
I hope this helps!