Hello, Daniel! Thank you for doing your homework! Have a great weekend! :)

1. cashier , Excuse me, where's the cashier
CORRECT: Excuse me, where's the cashier?

2. librarian , I turned my library books over to the librarian
CORRECT: I returned the library books over to the librarian.

3. vet , My puppy is sick and took care of it was a vet
CORRECT: My puppy was sick and the vet took care of it.

NOTE: Please don't forget to use punctuation. :)