This is a complex sentence.
A complex sentence consists of an independent clause and one or more dependent clauses. We also have 2 clues: a relative pronoun: that, who, which etc. and a comma Let's break it down.
Dependent clauses don't make sense on their own. Yes, they have a subject and a verb, but if you went up to someone and said, "who has been studying medicine for the last 3 years" then turned around and walked away, the person would think you were some kind of a weirdo.
Independent clauses, on the other hand, can stand alone and make sense all by themselves. They too have a subject and a verb. Now, if you took OUT the dependent clause and went up to someone and said, "Paula will work at the new clinic next summer". They may still think your weird, but at least they would understand what you were saying.
Now why ISN'T it a simple sentence:
Simple sentences consist of just one independent clause and express a complete thought. So if the sentence was "Paula will work at the new clinic next summer". Then it would be simple sentence.
For the future? Look for your clue words: who, that, which, and your commas...
Hope this helps!
Student or Learner