Okay.
So we’ve got an entirely flat surface that also happens to be the exact same length as the earth’s surface.
If you had one continuous piece of string that went from one end of that flat surface to the other, and on one end there was attached a bell… would you be able to ring the bell by pulling on the other end of string?
Distribute the pi on the right side of your equality, and replace c with pi*d:
c+x = pi*(d+2)
pid+x = pid + pi*2
x = pi*2
To generalize for an height h,
x = pi2h
Edit: I did some weird markup, but won’t be fixing it
Oh, I see now. I missed some pretty basic math there with that distribution. That makes sense now, thank you!