Elon Musk is offering to buy Twitter outright, just days after the Tesla CEO said he would no longer be joining the social media company’s board of directors. Twitter Inc. said in a regulatory filing on Thursday that Musk, who currently owns slightly more than 9% of its stock and is the company’s biggest shareholder, provided a letter to the company on Wednesday that contained a proposal to buy the remaining shares of Twitter that he doesn’t already own. Musk offered $54.20 per share of Twitter’s stock. He called the nonbinding proposal his best and final offer.