5 would be "preparé," because you are preparing the sandwiches. Preparar means "to prepare." When you conjugate preparar in past tense, you should know that verbs followed by "yo" should end in "é."
6 is ayudamos. The verb is ayudar, because that means "to help." The conjugation for past-tense nosotros is the same as present-tense, so it would just be ayudamos.
This is a tricky concept to understand, so I hope I explained it well enough!
