We are supposed to complete the sentence with the correct form of words provided, and then translate into English. It is using reflective verbs, and I do not understand how to do them.

Answer:
Warum ärgerst du dich?
In English: Why are you upset?
Ina freut sich auf die Reise.
In English: Ina's looking forward to the trip.
Explanation:
1. Look who you are talking to. "Du"(Pronoun) is singular.
Ich ärgere mich
Du ärgerst dich
Er/Sie/Es ärgert sich
2. You are talking about Ina. He/She/It is looking forward to the trip.
-> Er/Sie/Es freut sich.