When was algebra first developed?
History of algebra
1637
Mathematical historians[77] generally agree that  the use of x in algebra was introduced by René Descartes and was first published in his treatise La Géométrie (1637).[78][79] In that work, he used letters from the beginning of the alphabet (a, b, c,...) for known quantities, and letters from the end of the alphabet (z, y, x,...) for unknowns.[80] It has been suggested that he later settled on x (in place of z) for the first unknown because of its relatively greater abundance in the French and Latin typographical fonts of the time.[81]