Hey littlejon.
You have to group the values into "bins" and then find their frequencies.
After you do this, you make something a probability distribution by dividing each of the frequencies (for each bin) by the total number of all frequencies [i.e. the sum of frequencies].
Do you see that there are only 6 possible outcomes for (x, y): (1, 1), (1, 2), (2, 1), (2, 2), (3, 1), and (3, 2)?
What are the probabilities for each of those?
And there are four possible outcomes for X+ Y: 2, 3, 4, and 5. 1+ 1= 2; 1+ 2= 2+ 1= 3; 2+ 2= 3+ 1= 4, and 3+ 2= 5. What are the probabilities of each of those?
If U= 4, then (x, y)= (2, 2) or (3, 1). So x has two possible outcomes, 2 or 3. What are the probabilities of x= 2 or 3 given that (x, y)= (2, 2) or (3, 1).