Well, you first need two Slowpoke and a king's rock. Give the kings rock to one and trade it. When you get it back your Slowpoke shall evolve into a Slowking.
Take the king's rock from your Slowking and give it to the other Slowpoke. Trade again and, tada!! You have two ^^
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king's rock evolves poliwhirl into politoed and slowpoke into slowking when they are traded
Have your Slowpoke hold a King's Rock, then trade it with a friend to get a Slowking.
You can't catch any of Slowking's line in Hoenn. You must trade it from LG.
You can't. Slowking and Politoed are generation 2 pokemon, and Yellow is a generation 1 game, so Slowking, Politoed, AND kings rock didn't exist at the time.
Slowbro does not evolve to Slowking in order to get a Slowking you must trade a Slowpoke to a friend while it holds the item King's Rock and after the trade is completed the Slowpoke will become a Slowking. Have your friend send the Slowking back to you in order to recieve the data on it for your pokedex.