Why? It is exactly the same as the question in the GMATPrep software. Is it because of the 2z there, because we know 2z is always even, so why did they put it there?
The question is really equivalent to Is x+y even?
Here each statement alone is insufficient.
1) x = 3, z = 3, y = 4 Answer is No
x = 2, z = 2, y = 4 Answer is Yes.
same logic for statement 2 as well.
Now when we combine the two statements, do you see a clean way to do it that involves a simple step.
No paper or pencil required.
Free Video Explanations: OFFICIAL GUIDE FOR GMAT REVIEW 2015.