You can
put this solution on YOUR website!Note: the tick above the set name simply means the complement of the set.
A′ = Everything in set U but NOT in set A = {18, 19, 20, 22, 24, 26, 28}
B' = Everything in set U but NOT in set B = {18, 19, 20, 21, 22, 23, 24, 25}
Now simply combine the two sets A' and B' (and remove any duplicate elements) to get
A′ U B′ = {18, 19, 20, 22, 24, 26, 28} U {18, 19, 20, 21, 22, 23, 24, 25} = {18, 19, 20, 21, 22, 23, 24, 25, 26, 28}
So the answer is
A′ U B′ = {18, 19, 20, 21, 22, 23, 24, 25, 26, 28}