Hi AbeNeedsAnswers,

We're asked to find the value of angle X in the parallelogram.

When dealing with a parallelogram, there are a variety of math rules about that shape that you might find useful:
1) Opposite sides have equal length
2) Opposite angles are equal
3) The two angles on a 'side' of the shape will add up to 180 degrees.

1) Y = 2X

Since angle Y and angle X are on the same 'side', we know that Y + X = 180 degrees. With Fact 1, we are given a second equation to work with, so we clearly have a 'system' of equations (so you can figure out the values of X and Y). In this case, X = 60, Y = 120
Fact 1 is SUFFICIENT

2) X + Z = 120

Since X and Z are opposite angles, we know that they are equal. With the information in Fact 2, we now know that they are X = 60 and Z = 60.
Fact 2 is SUFFICIENT

Final Answer: D

GMAT assassins aren't born, they're made,
Rich