Take a look at the sentence again:
Hoping to relax after a busy week at work and school, Cheryl smoothed on
some sunscreen, settled onto her pool raft, and
opened her new Stephen King novel, then her neighbor cranked up the lawnmower,
ruining the quiet.
The comma between raft and and is connecting the second item in a series, settled onto her pool raft, with the last item in the series, opened her new Stephen King novel. This comma is legal.
Go back to the sentence to try again.