Complex sentence: Frustrated by Champ's curiosity Zach unlocks one of the books, but when Hannah discovers them, Zach drops the book, releasing the Abominable Snowman of Pasadena.
Split into simpler sentences: Frustrated by Champ's curiosity, Zach unlocks one of the books. When Hannah discovers them, Zach drops the book, releasing the Abominable Snowman of Pasadena.