Your french is good apparently , but for me , i don't know a single word of Hebrew
Here is the file http://www.animath.fr/IMG/pdf/cours-arith1.pdf see page 68-.. (i hope that the mods and admins don't mind )
I don't really get it too !
Because using Wolfram i only get the solutions i found in my first answer ! really weird !
Here , take a look http://www.wolframalpha.com/input/?i=y^2%3Dx^3-5x
I have to go now , i hope we can discuss this later