This question was closed as off-topic. It's in the middle of being re-opened, but there's no point in fighting to close or re-open until a consensus is reached.
It can't be answered because of a fundamental flaw. In short, it asks answers to calculate the area of a city, but only says how thick the perimeter is, leaving the area contained ambiguous.
However, math is on-topic here.
We have a tag devoted to it, mathematics, as well as physics which often overlaps. We have questions devoted to orbital-mechanics, and dozens of hard-science questions relying on mathematical formulas for answers.
Why is finding the area of a polygon any different from calculating how structurally strong an object would need to be to survive a trip through an ergosphere, or how to get two asteroids to follow each other in stable ellipses?
Don't get me wrong. The question is flawed, and an answer could point that out, but I don't think it's any less fit for the site than other mathematics questions with worldbuilding applications.
Math is math.