STARDOM - 0 this rules out A as the 3rd letter
REDOING - 2
UNDOING - 1
That rules out R as starting letter since E counts as 1, and dropping the RE from REDOING-2 would have had UNDOING as 0 if R were correct. Since UNDOING had 1, then R is not correct and therefore is ruled out.
STARDOM - 0 rules out R as 4th letter.
So for REAREND, 3 letters are ruled out, and since it has 4 points, that means -E--END are correct, as the only letters left.