i believe it is the word traigo that must fit in the blank
Estábamos sirviendo would be the answer for the nosotros form in the imperfect progressive. For future reference, you should check out the Spanish Dict online, it has all of the verbs in every form and tense :)
If Pablo says...
Pablo: [Trabajé] como cajero en el restaurante de comida rápida.
But if it's a description of Pablo...
Pablo [trabajó] como cajero en el restaurante de comida rápida.
Tú vas would be the answer
