Past tense: The Dobbs file a complaint with the main office, and a vice president has offered to personally investigate the problem.

The answer is "The Dobbs filed a complaint with the main office, and a vice president had offered to personally investigate the problem." You must change the verb in order to change the sentence into a past tense sentence. The verb in the sentence are "file" and "has", therefore you must change them to "filed" and "had".