6. My friends (not to do) their homework at seven o'clock yesterday. They (to play) volleyball.

6. My friends weren't doing their homework at seven o'clock yesterday. They were playing volleyball.

В данном предложении нужно использовать Past Continuous, так как есть указание на время в прошлом (yesterday - вчера) и указание на процесс (at seven o'clock - в семь часов). Past Continuous используется для описания действий, происходивших в определенный момент в прошлом.

Ответ: My friends weren't doing their homework at seven o'clock yesterday. They were playing volleyball.