Question 1104722
my diagram from your picture is shown below:


<img src = "http://theo.x10hosting.com/2017/121805.jpg" alt="$$$">


step 1 is as shown in my diagram.
step 2 is as shown in my diagram.
step 3 reason is shown as A.
step 4 reason is shown as B.


A.  corresponding sides of congruent triangle are congruent.


B.  If a quadrilateral has one set of opposite sides which are both congruent and parallel, then it is a parallelogram.


statement B pertains to SV and TU in the diagram.


here's a reference on theorems dealing with parallelograms you might find useful.


<a href = "https://mathbitsnotebook.com/Geometry/Quadrilaterals/QDParallelograms.html" target = "_blank">https://mathbitsnotebook.com/Geometry/Quadrilaterals/QDParallelograms.html</a>


look to the bottom of the above reference to see the theorem used in proving your problem.


the proof of theorem mention in B is referenced below.


<a href = "https://mathbitsnotebook.com/Geometry/Quadrilaterals/QDThPf10.html" target = "_blank">https://mathbitsnotebook.com/Geometry/Quadrilaterals/QDThPf10.html</a>