Elon Musk’s $21 billion mystery: Where will he get cash for Twitter?

Elon Musk and Twitter Inc. reached an agreement for the world’s richest man to buy the social networking platform for $44 billion, resolving the pressing question of whether the company’s board would consent to the leveraged buyout deal.

On Musk’s side, though, there remains a mystery: How is he going to cover the $21 billion equity portion of the transaction that he’s personally guaranteed?

Musk, 50, has outlined the $13 billion in bank financing secured by the social-media company and the $12.5 billion backed by a pledge of some of his $170 billion Tesla Inc. stake. But he’s been short on details about how he’ll fund the remainder.

“It’s serious,” Steven Davidoff Solomon, a professor at the School of Law at the University of California, Berkeley, said of the new filing. “He’s getting more professional and this is starting to look more like a normal hostile bid. You don’t do that unless you’re going to launch an offer.”

In a tender offer, otherwise known as a hostile bid, Mr. Musk would take his bid directly to Twitter shareholders without the consent of the company’s board. While Mr. Musk has not publicly outlined a business plan for Twitter under his ownership, he offered a glimpse of what he might do on Thursday.

Mr. Musk responded to an emailed request for comment by writing, “X.”

A Twitter spokesman confirmed that the company had received Mr. Musk’s updated proposal and reiterated that its board was “committed to conducting a careful, comprehensive and deliberate review to determine the course of action” that would be best for the company and its shareholders.