Final answer:
The fill in the blank requires the verb 'estar' to be conjugated in the future tense as 'estaremos'
Step-by-step explanation:
The correct form of the verb estar in the future tense to complete the sentence 'Elena and yo ____ en la fiesta' is estaremos. This is the first person plural form of the verb, which indicates that Elena and the speaker will be at the party in the future.
For example, you could say 'Elena y yo estaremos en la fiesta el sábado' which translates to 'Elena and I will be at the party on Saturday'.
Learn more about Conjugating verbs in Spanish