What mathematician was the first to use X to represent an unknown ?

It was Renee Descartes, the 17th century philosopher and
mathematician, who began the use of letters at the
end of the alphabet to represent unknown quantities, with X as
the choice for the first unknown.

He is also credited with both the the Cartesian graph, which we all know and love as either the (x,y) graph or the (x,y,z) graph, and also the famous phrase "I think, therefore I am."