Answer: n'a pas regardé
Explanation: The passé composé is formed using a helping verb, which in most cases is avoir, and a past participle. In this case, the helping verb is avoir and the past participle is regardé. Negation also comes into play here because you're writing a negative sentence. Remember the double negation in French, which means you have to have both "ne" and "pas" if you're going to put something into the negative.
Going back to the passé composé, "regarder" would become "a regardé", using the helping verb and participle to form that. From there all you have to do is put the double negative on it. Remember that the double negative goes around the verb. Using that, you end up with "n'a pas regardé".