Find X: Converting Quadrilateral ABCD into a Parallelogram with Side Length 18

The problem requires determining the value of $x$ for the quadrilateral ABCD to be a parallelogram, which involves setting opposite sides of the quadrilateral equal to each other.

For a quadrilateral to be a parallelogram, its opposite sides must be equal. Therefore, we can set the following equality:

$BC = AD$

In the given diagram, it states that $BC = 18$ and $AD = x$. Since opposite sides of a parallelogram are equal, we have:

$x = 18$

Thus, by equating the lengths of the opposite sides, we derive the value of $x$ as $\textbf{18}$.

Therefore, the value of $x$ for quadrilateral ABCD to be a parallelogram is $x = 18$.