I have had difficulty understanding this topic with Union and Mutually exclusive involved. Can anyone explain what is the N( X U Y) and the N ( X *upside down U* Y) is? the book is confusing me because they are using it interchangeably

Also I have a question on my hw (I will use the letter i for the upsidedown U)

Suppose two fair dice are rollled, Find N(X U Y) and N(X i Y)

X=the first die shows 6

Y=the sum of the dice is 10