Just for kicks, I'll use my arclength tool, see how well it can do.
That does not help you any, but is it close? There are at least 3 ways I can think of here. Let me see, if I compute the proper arclength differential, call it L...
Oh well, at least that gives you the right answer to check.
Can you compute that integral exactly? Hmm, well, yes. ... The answer is:
And as a decimal, that is...
Again, you need to do your own homework, but that appears to be the analytical solution. When they cooked up the problem, it was one where things simplify down nicely. You could probably do it using pencil and paper. Anyway, at least this gives you a target, so you know when you got it right.