"The intersection (∩) of a pair of sets (G and H) is a third set (I) composed by the elements that belong, at the same time, to both given sets."
According to this definition:
Given the sets:
G = {3, 7, 8, 9}
H = {2, 5, 7, 8}
The Intersection is:
G ∩ H = I = {7, 8}
:-)