They have provided that x,y and z are three non negative integers(basically, whole numbers)
Therefore, either x or y or z will be ==0.
So, z can be equal to 0 or 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10.
Among those events, 6 of them are even (,0,2,4,6,8,10)
since 11 is the total possible outcomes.
Probability(z is even)= 6/11