Final answer:
Without the source material or specific context, we cannot identify what car Mr. Dern intends to buy with the extra money from overseeing detention, as this question relates to content from an English literature assignment.
Step-by-step explanation:
The question does not specify which particular car Mr. Dern is planning to buy with the extra money he earns from overseeing detention. This question appears to be referencing a narrative or a story, possibly from an English literature assignment or from a character's statement in a novel, play, or short story. Generally, English literature questions delve into plot details, character analysis, thematic elements, or interpretative discussions based on a given text. Without more context or the source material, it is not possible to provide a specific answer to what car Mr. Dern intends to purchase.