For the first sentence, "Kayla and Jacob were rivals in the swimming contest." The word rivals means they are enemies and are competing against one another in the contest. So it has a negative connotation.
The second sentence, "Leah was known for her mysterious plots." By mentioning that she is "known for" her plots, it implies she is notoriously famous for her plots. Also, someone who "plots" is not normally described as a positive person, so the answer to both is negative.
