Complex sentence: In 1738, he was created Cardinal Priest, and three years later, he became pro-librarian of the Vatican Library to Cardinal Angelo Maria Quirini, whom he would succeed him as librarian in 1755.
Split into simpler sentences: In 1738, he was created Cardinal Priest, and three years later, he became pro-librarian of the Vatican Library to Angelo Maria Quirini. Cardinal Quirini he would succeed him as librarian in 1755.