"She works at this university."
Translation:Ella trabaja en esta universidad.
My understanding is that using 'a' is generally like English 'to' and 'en' is equivalent to 'in'. (Although, both words have multiple uses/meanings depending on context. For example, 'a' can also mean 'at' when talking about a specific moment in time and is also used as the 'personal a', amongst others).
Ella trabaja EN una oficina.
- She works IN an office.
Ella va A la oficina.
- She is going TO the office.
For all the uses of 'a' you can check out the entry on SpanishDict: www.spanishdict.com/translate/a
As a side note, like many verbs, 'ir' requires the preposition 'a' in front of a noun or infinitive. A list of verbs requiring prepositions (por, en, de, con, and a) can be found here: www.lawlessspanish.com/grammar/verbs/verbs-with-prepositions/.
Este is masculine, so when Universidad come up, you use esta because it is feminine. But I have a Hispanic friend and he never uses esta. ( I think that duolingo is teaching us how to speak proper, regular Hispanic people speak in Spanish slang like we speak in English slang.)