The contraction "weren't" is formed by combining the words "were" and "not".
To determine the words that form the contraction in the sentence, you can identify the contraction by finding an apostrophe and then look at the words on either side of it. In this case, the contraction is "weren't." The words that form the contraction are "were" and "not." So you can fill in the blank space with "were" and "not."
