(Mishnah): Four death penalties were given to Beis Din. [The order from most severe to least severe is] Skilah (stoning), Sereifah (burning), Hereg (beheading through a sword), and Chenek (choking);

R. Shimon says, Sereifah, Skilah, Chenek and Hereg.

This (the previous Perek) taught Mitzvas ha'Niskalim.

(Gemara): The kingdom is authorized to kill only through Hereg.

What is the source for Skilah? "U'Skaltam ba'Avanim va'Mesu." We learn Sereifah from "Yisrefu Oso v'Es'hen."

[The source for] Hereg - it says here Nekimah, and it says below "v'Heveisi Aleichem Cherev Nokemes Nekam Bris." Just like Nekimah said there is the sword, also Nekimah said here is the sword.

We do not find [an explicit] source for Chenek. You learn from here that any Stam Misah mentioned in the Torah, you cannot be stringent about it, rather, you are lenient, and attribute it to Chenek (the lightest Misah).