Where ..... you yesterday, Sam? – I ..... at the zoo with my mum.

1. The first blank is part of a question asking about the location of "you" yesterday. The past tense of "to be" for "you" is "were".
2. The second blank is the answer to the question, referring to "I". The past tense of "to be" for "I" is "was".
Answer: were, was