Complex sentence: She learns that Johanna and Peeta were taken away by the Capitol and attempts to kill Haymitch because he didn't fulfill his promise, but is injected with a serum that allows her to fall asleep by Finnick.
Split into simpler sentences: She learns that Johanna and Peeta were taken away by the Capitol, causing her to attempt to kill Haymitch because he didn't fulfill his promise. However, she is injected with a serum that allows her to fall asleep by Finnick.