Am I stupid? (What is the circumference?)

Please, tag anwsers as spoilers before posting, I still didn't crack it.

@Inginsub
Fuck me, I didn't see it without a paper.

And I had to use an actual equasion. Mark one side (and middle) as 9+x and the second as 9-x
So... 30

@Inginsub
Well, this is what a 3rd grade looks like (supposedly) these days in Czech Republic.