A big problem for smart people

I have two possibilities; X1 and X2.
The question is: If you know that X1 gives you 0 then what does X2 give you?

I set up a table:
X2: 2 0 -2
0 0,1 0,2 0,1
-2 0 0,3 0,3
Then I found:
Px1(0) =0,4, Px1(-2) = 0,6
Px2(2) = 0,1, Px2(0)=0,5, Px2(-2)=0,4
I know I´m supposed to use a formula for „conditional expected value“ and the formula is pX|Y=y(x) = p(x,y)/(„sum of x“ p(x,y))
but I do know understand how to do that or what „sum of x“ is or what p(x,y) is.
please help!!