In this chapter we present the Libro de Algebra en Arithmetica y Geometria by Pedro Nuñez (1567), a very interesting treatise written, according to the author, to understand the new algebra or “Arte Mayor”. It must be considered that in the sixteenth century algebra was little known in Spain and, as in other countries, had to find its place between consolidated arithmetic and geometry. We will explain the different parts of the work and remark on some innovations that the author includes in relation to the other treatises of the time. It contains arithmetical and geometrical problems solved by algebra and the demonstration of the rules for solving equations with geometrical tools. One of the main objectives of the work is to convince the reader of the importance of learning algebra because it is a powerful tool for solving both arithmetic and geometric problems. The original title of Book of Algebra in Arithmetics and Geometry. This research is included in the project: PID2020-113702RB-I00.