Since "Grammar Builder 2" is a popular title used in various educational contexts (most notably the series by Cambridge University Press, often used in Singapore and ESL contexts), and an "answer key" is a functional document rather than a narrative text, the best approach for an essay is to analyze the role and significance of the answer key within the language learning process.
For those looking for the answer key specifically, it is generally included within the book itself or available through academic platforms:
The Grammar Builder 2 Answer Key New supports this by including a self-assessment log in the appendix.