Take a look at the passage again:
While Kristy labored through another evening of incomprehensible accounting
homework, her roommate Bob read a sci-fi novel. And ate an entire bag of
potato chips.
You chose to connect the lonely verb fragment to the main clause in front, like this:
While Kristy labored through another evening of incomprehensible accounting
homework, her roommate Bob read a sci-fi novel
and he ate an entire bag of potato chips.
Unfortunately, removing the period and adding the subject he won't do the trick unless you also remember to put a comma after novel. Review the rules.
Go back to the sentence to try again.