Who used first the letter "X" as an unknown in algebra?

Diophantus was the first to use an "unknown".

I have no idea if Diophantus used letters, which ones,and if he did ,they certainly were from Greek alphabet.His work was translated in many languages later on.Good guess Mommy"X"2.

But I am looking for someone from the 1600's.

Upon further thought, I think Diophantus used an empty square. I don't know anything about the Greek alphabet, that may be a letter for all I know, but I remember that symbol when I learned about the development of algebra.

No idea on who used "x", but I think I may owe them royalties.

René Descartes (1596-1650) might be the answer you're looking for, though I can't quite nail it down.

According to Mathematics For The Million by Lancelot Hogben (Norton & Co., 1937) -- a moldy oldie which I purchased for 25¢ at a thrift shop not too long ago -- specifically Chapter VII ("How Algebra Began"):

quote:

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Mathematicians use the term "algebra" to mean rules for solving problems about numbers of one kind or another, whether the rules are written out in full (rhetorical algebra), or more or less simplified by abbreviations (syncopated algebra), or expressed with the aid of letters and operative signs exclusively (symbolic algebra)...the shorthand first used by Descartes was generally adopted, and mathematics was liberated from the clumsy limitations of everyday speech.

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You wouldn't believe what I had to plow through to find the passage above. Despite the fact that the author taught at the London School of Economics his prose is hardly economical. At any rate he gives an example of the quadratic equation 3x² - 5x + 6 = 0, attributing this specific notation to Descartes ca. 1637.

René Descartes is the answer I was looking for, Professor. I first found this answer on a trivia site,(but you can't always trust them).
I also found the same answer in David Feldman's book "How do astronauts scratch an itch" which is also their source. Source

Trust only AnswerPool.com Trivia.