this is where i download unity for it http://dev-io.ro/unity3d-5-3-1-build-for-linux-now-available/
when i finish i come a cross a problem that every time i start unity it show recent grey screen and i leave my laptop on for hours and even restart my laptop it didnt change anything but as soon as i download NPM it work to download NPM go to the terminal and write sudo apt-get install npm to download it if you come across more bugs go to this website https://forum.unity3d.com/threads/starting-into-black-recent-screen.384332/