for the figure to be a parallelogram, opposite angles need to be congruent
that means B = D and A = C
thus [tex]\bf \begin{cases}
3y-20=2x+6\\
-2x+3y=26\\
----------\\
4x=4y+4\\
4x-4y=4\\
x-y=1
\end{cases}\implies
\begin{cases}
-2x+3y=26\\
x-y=1
\end{cases}[/tex]