The use of z,y,x to represent unknowns is due to Rene Descartes in his La geometrie. He introduces the use of the first letters to signify known quantities while he used last letters to signify unknown quantities. He used the last letter z for the first unknown and proceeded backwards to y and x for the second and third, respectively.
The predominant use of the letter x to represent an unknown value came in an interesting way. During the printing of Descartes’ La Geometrie which introduced coordinate geometry, the printer reached a dilemma. While the text was being typeset, the printer began to run short of the last letters of the alphabet. The publisher asked Descartes if it mattered whether x, y, or z was used in each of the book’s many equations. He replied that it made no difference. The printer selected x for the most unknowns, since the letters y and z are used in the French language more frequently than is x.