Preterite be imperfect question

Hi, I was hoping someone could help me out! I am trying to write the sentence: “when we arrived at the airport, we checked the luggage”. I have “Cuando ____ al aeropuerto, facturamos el equipaje”. I have no idea if llegar is supposed to be preterite or imperfect… I was taught that if the event ended then it is preterite but I am not sure in this case. If anyone could answer and explain it would be great. Thank you!!

submitted by /u/loosecriminal
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