The faulty parallelism is caused by the "he was" in the second clause in the sentence. The corrected version is: Jefferson was both the writer of the Declaration of Independence and the nation's third president.
Explanation:
A faulty parallelism results when a sentence or list is not structured so the grammar is parallel. This affects the flow and syntax of the sentence making it harder to understand or read. This error is also known as a parallel structure error or a parallel construction error. For lists, you want to assure that each point begins with a noun, or each point begins with a verb and not a mixture of different parts of speech. You also do not want to mix gerunds with infinitives, for example, when listing your skills or qualities in a sentence on your CV.
