1) n(a) = 5
The cardinality of a set is the number of elements in that set. So, you simply need to count how many elements belong to the set, and the answer is 5
2) n(b) = 3
Simlarly, in this case you need to count how many elements belong to the set, the answer is 3
3) n(a ⋃ c) = 6
The union of two set is a set composed by all the elements belonging to either one of the sets, with no repetitions. So, since the first set contains all numbers from 1 to 5, the second set contains a 4, which however already belonged to the first set, but also a 6, which is added in the union. So, a ⋃ c = {1,2,3,4,5,6}, and thus n(a ⋃ c)=6.
4) n(a ⋂ c) = 1
The intersection of two sets is a set composed by the elements belonging to both sets. So, 1,2,3 and 5 don't belong to the intersection, because they belong to the first set alone, while 6 doesn't belong to the intersection because they don't belong to the first one. So, a ⋂ c = {4}, and thus n(a ⋃ c)=1.
