This talk will describe the author’s work translating a paper of Euler on number theory, and how researching the history of this paper led back to Lagrange, Fermat, and ultimately to a geometry problem of Diophantus. We will investigate Euler’s algebraic creativity and Lagrange’s explanation of his own method, inspired by Fermat. Moreover, we will demonstrate one way in which this paper of Euler’s has been used as part of a project in an undergraduate history of mathematics course. We will even investigate the continuation of Euler’s work.