The intersection of two sets [tex] A [/tex] and [tex] B [/tex] is the set of all elements belonging to both [tex] A [/tex] and [tex] B [/tex].
So, in this case, the intersection between [tex] P [/tex] and [tex] Q [/tex], which we write as [tex] P \cap Q [/tex], is the set
[tex] P \cap Q = \{a,c\}[/tex]
The union, which we write as [tex] P \cup Q [/tex], is the set of the elements belonging to at least one of the sets without repetitions. So, you can simply list one set and add the elements of the other which didn't already belong to the first one:
[tex] P \cup Q = \{a,b,c,d,e,f,g,h,i\}[/tex]
Finally, with [tex] n(A) [/tex] we indicate how many elements belong to [tex] A [/tex], so you can simply count them:
[tex] n(P \cap Q) = 2 [/tex]
[tex] n(P \cup Q) = 9 [/tex]
