B'=3,6,7
Complement of a set
-complement of a set A,written A',is the set of elements in the universal set that are not in A.
Example
U={1,2,3,4,5}
A={2,4}
B={2,3,4}
C={1,2,3,4,5}
D={}
Answers in the example
A'={1,3,5}
B'={1,5}
C'={}
D={1,2,3,4,5}
uhm hope it helps :)