Since $\overline{VW} \parallel \overline{UX}$, we can use the Triangle Proportionality Theorem which states that if a line parallel to one side of a triangle intersects the other two sides of the triangle, then the line divides these two sides proportionally.
So we can set up the proportion:
$\frac{YV}{XY} = \frac{VU}{XW}$
We know that $YV = 18$ and $YV + XY = 39$. So, $18 + XY = 39$.