Hi! Can you try sending a picture of the figure that this is referring to, so I can try answering the question?
To find the circumference of a round object, you will take the diameter and multiply is by pi to get 78.5mm
Polygons are similar? Well there is no picture provided, so I have no idea what you're talking about. But I think you might be asking if in general all polygons are similar, and if that's the case, they are not. Similar means they are similar in at least one aspect, and all polygons are different from each other, only adding one line for each bigger polygon. So I don't know if this helped you, but I tried anyways.
Your percentage of 97.6% as a decimal is 0.976
