Junior threw his geometry book when he saw the name "Agnes Adams" inscribed in it. Agnes Adams was not just a previous user of the book—she is Junior's mother.
When Junior did the math, he realized that the book he was holding had to be at least 30 years older than him. This made him both sad and angry, as he realized that thanks to the overwhelming prevalence of poverty in his community, he had to study from a book that was hopelessly out of date.
Thanks to this, Junior felt certain that his hopes and dreams would all come to nothing. This action suggests that Junior is angry about the state of his community and is unwilling to sacrifice his future because of the poverty faced by his community.