what is missing?
La profesora va______ preparar un examen
When someone is going to do something in Spanish, one uses "ir + a + infinitive."
Voy a hacer mi tarea. I am going to do my homework.
Va a dormir. He is going to sleep.
Vamos a cantar. We are going to sing.
What do you think is missing in your sentence?
I agree with dandi, but when speaking you might not listen that extra /a/ since is joined with va by making a slight long vowel sound.
"a"? Are you asking because you don't know, or because you want to see if others know?