Descartes published La Géométrie in 1637 as an appendix to his book on reason in science, Discourse on the Method of Rightly Conducting One’s Reason and of Seeking Truth in the Sciences.
One of Descartes’ most enduring contributions to mathematics was the development of the x and y axes. This system is known as Cartesian coordinates (Cartesian comes from Descartes).
He was also the first to use x, y, and z to denote variables, and a, b, and c to denote values that are known.
He also pioneered the use of a superscript 2 to denote “squared” and a superscript 3 to denote “cubed” (i.e. x² and x³). This can be seen in old-timey French here and in translated English here (in the 5th paragraph on the right page).