What is "?"

[Matrix]

"A ∈ B" means "A is an element of B". In this case it means x is either 4 or -2.

By the way, these is Wikipedia for these kind of questions:
http://en.wikipedia.org/wiki/∈

 

[DMOC]

Thanks for the link, I checked wikipedia but did not know what to type in the search function box, probably should have just copied and pasted that symbol.

 

[Rashiminos]

"A ∈ B" means "A is an element of B". In this case it means x is either 4 or -2.

By the way, these is Wikipedia for these kind of questions:
http://en.wikipedia.org/wiki/∈

Holy shoot, no. x can be any value between -2 and 4, inclusive.

Denoting a set is {4, -2} , x=4, xor x=-2
Denoting an interval is [-2, 4] , -2 <= x <=4

BIG difference, especially with a function.

(Plus elements of sets are usually lowercase in standard set notation)

 

[Leifmk]

Denoting an interval is [-2, 4] , -2 <= x <=4

And if one of the end points were to be excluded, the notation would use ( or ) instead, for example [-2, 4) means -2 <= x < 4.

 

[Matrix]

I stand corrected. I didn't know the difference between [...] and {...}.

 

[raketooy]

And if one of the end points were to be excluded, the notation would use ( or ) instead, for example [-2, 4) means -2 <= x < 4.

I have always been taught to use outward-pointing square brackets. So -2 <= x < 4 would be [-2, 4[.

 

[ParadigmShifter]

That is acceptable too. [x,y) same as [x,y[

 

[Leifmk]

I have always been taught to use outward-pointing square brackets. So -2 <= x < 4 would be [-2, 4[.

Huh, that was a new notation for me. Mathematical notation does have its little dialects, I know; there are often some differences between practice in countries most influenced by Anglophone or Francophone or German academic traditions, and so on. And obviously habits change over time; it's been over a decade since I did any real math now.