By definition, a parallelogram is a quadrilateral with opposite sides parallel and equal.
Since the opposite sides are equal, the following equations are true
[tex]\begin{gathered} 5x=3x+18 \\ 3y-7=2y+4 \end{gathered}[/tex]
Solving the first equation, we have
[tex]\begin{gathered} 5x=3x+18_{} \\ 5x-3x=18 \\ 2x=18 \\ x=\frac{18}{2} \\ x=9 \end{gathered}[/tex]
Solving the second equation, we have
[tex]\begin{gathered} 3y-7=2y+4 \\ 3y=2y+4+7 \\ 3y-2y=11 \\ y=11 \end{gathered}[/tex]
And those are the solutions.
[tex]\begin{cases}x=9 \\ y=11\end{cases}[/tex]
