Since it is a parallelogram then it is true that
That is, the segments are congruent
[tex]\begin{gathered} AE\cong EC \\ DE\cong EB \\ AD\cong BC \\ AB\cong DC \\ \end{gathered}[/tex]
So, in this case, you have
[tex]\begin{gathered} QT\cong TS \\ \text{ Then} \\ QT=21 \\ TS=21 \end{gathered}[/tex][tex]\begin{gathered} QS=QT+TS \\ QS=21+21 \\ QS=42 \end{gathered}[/tex]
Therefore, the length of QS is 42.
