"This country's coast is very very long."
Translation:ساحِل هٰذا الْبَلَد طَويل جِدّاً جِدّاً.
Consider it like this, "coast of this country is very very long".
Since هذا also makes the noun definite, it acts somewhat like ال in that regard.
So, consider it like "the coast of the(this) country" for translation purposes, but instead of just ال, you also add هذا before it.
Well, it's not quite true that you can't access the "tips" on a tiny mobile computer: you can, in fact, download the web version of Duolingo on it. I have both on my tiny mobile computer, and do the lessons on the app, but check the tips on the web version. Incidentally, I once learned to my cost that it is indeed only a tiny mobile computer, when my "phone" stopped working, and I found that the "phone" was only one of the many apps of this minute computer.
Something troubles my mind: Can I use a definite case in a possesive construct - like, in here, to write "السّاحل هذا البلد" - and if so, why isn't it used here? As far I understand, the alternative way to write this sentence in English is "the coast of this country is very very long.".
Also, since I'm already commenting: why does "beach" not accepted as equivalent to "coast: in this lesson?