Explanation:
∠YXZ and ∠VXU are vertical angles, and therefore are congruent.
∠YXZ ≅ ∠VXU, Vertical ∠'s ≅
Next, we're given that lines YZ and UV are parallel. That means that ∠YZV and ∠UVZ are alternate interior angles, and therefore congruent.
∠YZV ≅ ∠UVZ, If lines ||, then alternate interior ∠'s ≅
If two triangles have two pairs of congruent angles, then the third pair must also be congruent, making the two triangles similar.
△XYZ ~ △XUV, AA
Since the triangles are similar, then by definition, their sides are proportional.
XY/XU = YZ/UV, Definition of Similar Polygons