Here is the original item:
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 ate an entire bag of potato chips.
Unfortunately, you cannot connect these two parts with a comma. Review the rules.