Review the item:
When Millie overheard Coach Jones complain that women did not 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 did not 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.