Change each of these compound sentences into a simple sentence with a compound subject or object. gracie talked to william at school, and she talked to me at school.

A compound subject is when the word and is used to join the elements of the sentence. in order to make this a simple sentence, we need shorten it and take out the comma. the sentence will look like the following:
Gracie talked to William and I at school.
as you can see,, there is no comma being used after the and,, making it a simple sentence. the word and (written in bold) is used as the compound subject of the sentence as well.
let me know if you have any further questions
:)