Yep, in the tree that is described even in the example, this solution returns the incorrect value "15" instead of "-51".

Neat. I never knew that Python allows one line for conditional: resulting statement

I was wrong. Sorry

WRONG! Check "with your hands on paper" for n = 9. You cannot build 51 hexagons on 81 triangles! Only 49.

spoiler

flag.

Thank you and yes I understood it enough to solve the problem now. Super cool!

Added. Hope this will be understandable

Maybe adding this to the description :

<svg xmlns:osb="http://www.openswatchbook.org/uri/2009/osb" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 127.219 110.614" height="418.069" width="480.829"><defs><linearGradient osb:paint="solid" id="a"><stop offset="0" stop-color="#00df00"/></linearGradient><linearGradient gradientUnits="userSpaceOnUse" y2="150.473" x2="118.61" y1="150.473" x1="55.382" id="b" xlink:href="#a"/></defs><path d="M93.458 89.647l-29.595 16.91-29.442-17.175.153-34.085 29.595-16.91L93.61 55.562z" fill="#00f8fc" stroke="#000" stroke-width=".185"/><path transform="matrix(.93938 -.55445 .54937 .94807 -100.327 -22.187)" d="M118.349 168.762l-31.515 18.007L55.48 168.48l.163-36.296 31.515-18.008 31.352 18.29z" fill="none" stroke="url(#b)" stroke-width=".197"/><path d="M123.851 106.281l-118.882.821L63.699 3.737z" fill="none" stroke="#ed0000" stroke-width=".338"/></svg>

Already have in description:

Let H(n) be the number of all regular hexagons that can be found by connecting 6 of these points.

I think a more helpful tip is to stress that the hexagons are formed by points and not lines.

@vlukyanets: just say it in the description (that the hexagons can have different orientations) otherwise you'll get hundreds of messages asking for this.

Searching 12th hexagon is way to solution

i also encountered the same issue, and i can't submit my attempt
although im 100% sure it's good
formula in reply (SPOILER)
btw, what formula is the tester using?