On a line graph, closed circles denote an endpoint of a graph where the point belongs to the set. An open circle denotes an endpoint where the final point isn’t a part of the set. Think of it like avoiding the very last point in the set, so that it isn’t equal to it.
20 is anything above negative six, where negative six isn’t a part of the set because of the open circle (A).
21 is numbers between negative four and five, where the two endpoints don’t belong to the set because of the open circles (C).
Hope this helps!