You'll need a ruler that has "centimeters" on it.
Use the ruler to measure the distance from Town Hall to the Art Gallery.
I don't have a ruler with me, so I can't say for certain what that centimeter distance will be. Even if I had a ruler, it won't be much use because I cant put it directly to the paper. Only you are able to do that part. But I'll go over a hypothetical example.
Let's say the on paper distance from Town Hall to the Art Gallery is 5 cm (roughly 1.97 inches).
This means we multiply both sides by 5 in the steps shown below.
1 cm on paper : 200 m in real life
5*1 cm on paper : 5*200 m in real life
5 cm on paper : 1000 m in real life
5 cm on paper : 1 km in real life
So the 5 centimeters measured out with the ruler corresponds to the real life distance of 1 kilometer. This would be the real life distance from Town Hall to the Art Gallery. Again, this is a hypothetical example. The actual centimeter distance is likely different and you'll need to get a ruler to measure it out for yourself.
Let me know if this helps. If you have further questions, then please let me know.