My professor said that Leibniz was not even aware of the concept. The Wikipedia page says that the formula was named "in honor of Gottfried Leibniz." What gives? Did he do work that was related, and if so, what did his formula look like?
This answer says that Leibniz was, by way of Gerolamo Cardano, but the Wikipedia article linked there has nothing on the subject.
Edit: Here is a historical article that has the following quote:
The preface to this contains the first notable historical sketch of the theory, and includes references to the writings of twelve outstanding mathematicians, beginning with Cramer (1750) and ending with the author's own contemporaries, Cayley, Sylvester and Hermite. In the same year (1850) there also occurred something out of the ordinary, for the correspondence between Leibnitz and the Marquis de I'Hôpital having been published from manuscripts in the Royal Library at Hanover, the striking discovery was made that more than half-a-century before Cramer's time the fundamental idea of determinants had been clear to Leibnitz, and had been expounded with considerable fullness by him in a letter to his friend.