Does anybody know why traditionally has been the letter $x$ used as the incognita in an equation? And then following back: $y, w, \dots$
Why starting from the end of the alphabet?
The convention of using letters at the front of the alphabet (a, b, c) for known cofficients and letters at the end of the alphabet (x, y, z) for unknowns originates with Descartes's La Géométrie from 1637.
I don't think it is known what inspired Descartes to exactly this convention, but the idea of using different groupings of letters for knowns and unknowns was already present with François Viète, whose works from the late 1500s were known to Descartes. Viète used consonants for constants and vowels for unknowns.