Answer
$QR$ = $3$
Work Step by Step
In congruent polygons, vertices are named in the exact order of their matching vertices.
If $WXYZ$ is congruent to $PQRS$, then vertex $W$ corresponds to vertex $P$, vertex $X$ corresponds to vertex $Q$, vertex $Y$ corresponds to vertex $R$, and vertex $Z$ corresponds to vertex $S$.
Therefore, $QR$ is congruent to $XY$, so $QR$ is equal to $XY$. If $XY$ is $3$, then $QR$ is also $3$.
