The quadrilateral ABCD is a parallelogram if both pairs of opposite sides are congruent. Given side lengths, prove that quadrilateral ABCD is a parallelogram by finding the value of x

In parallelograms, opposite sides are equal in length.
There are 2 pair of parallel sides.
In parallelogram ABCD, the congruent sides are:
• AB = CD
and
• AD = BC
Let's equate AB = CD and solve for x:
[tex]\begin{gathered} AB=CD \\ 2x+55=3x+35 \\ 55-35=3x-2x \\ x=20 \end{gathered}[/tex]Now, let's equate AD = BC and solve for x:
[tex]\begin{gathered} AD=BC \\ 40+\frac{x}{2}=3x-10 \\ 40+10=3x-\frac{x}{2} \\ 50=\frac{6x-x}{2} \\ 50=\frac{5x}{2} \\ 5x=100 \\ x=\frac{100}{5} \\ x=20 \end{gathered}[/tex]Thus, the x value is equal to 20.
Quadrilateral ABCD is a parallelogram.