These two verses are an expansion on verse 17’s “having first obtained mine errand from the Lord.” That statement was a preliminary giving the authority by which Jacob gave the sermon, but Jacob expands from that statement of authority to the general authority and exercise of that authority that he and Nephi had attempted. Both Jacob and Joseph were specifically consecrated as priests and teachers by Nephi, and both took their responsibilities seriously.
Jacob tells us that he was diligent in teaching his people, and next will provide one of his sermons as evidence of that statement.
Sociological: Both Jacob and Joseph were consecrated as priests and teachers. That Nephi is not specifically mentioned as a priest and teacher is interesting, but easily explained by his larger title of king, which would have included the ability to teach and administer (and clearly did allow him to preach, as indicated by the included sermon from 2 Nephi 31-32). That there were two priests and teachers again suggests a reasonably large population at this point in time. Had there been only a handful of people, one teacher would have been sufficient.
Historical: While “priests” and “teachers” are both descriptive of offices in the modern Aaronic Priesthood, we are not justified in seeing these terms in Nephite society in the light of modern Priesthood organization. These are merely descriptive terms, not offices. The Book of Mormon is not interested in priesthood organization at this point. It is sufficient that the Lord has given them the authority and responsibility. Because there are no competing concepts of how their religion should be lived (whatever the Lamanites had would be considered completely foreign) there is no reason to spend time defining priesthood against any other possible conceptions of that authority.