Take a look at the sentence again:
When Millie overheard Coach Jones complain that women didn't belong on a
basketball court, she became so angry that
she picked up a tennis racket and whacked him over the head, the assault
and battery charge, in Millie's opinion, was worth it.
That women didn't belong on a basketball court is a subordinate clause. A subordinate clause must be attached to a main clause. She became so angry is the necessary main clause. When a subordinate clause introduces a main clause, you need a comma between the two.
Go back to the sentence to try again.