Final answer:
Isaac should first use context clues to understand the meaning of 'proletariat' in his history textbook; if necessary, he may look it up in a dictionary or break it into parts for further understanding.
Step-by-step explanation:
When Isaac was reading his history textbook and encountered the word proletariat, the first strategy he should use to figure out the word's meaning is to look for context clues in the sentence. This method involves examining the words and sentences surrounding the unfamiliar term to infer its meaning. If the context does not provide enough information, Isaac might then consider looking the word up in the dictionary, breaking it up into parts to understand its roots, or trying to pronounce it out loud to see if it sounds like a word he is familiar with. However, using context clues is often the quickest and most immediately accessible strategy.