You haven't provided the parallelogram , therefore, I cannot give an exact answer.
However, I will help you with the concept.
When reflecting a point across the line y = x:
We swap the values of x and y while keeping their sign the same.
This means that:
(x,y) ............> reflection across y = x ..........> (y,x)
Examples:
(2,3) ..............> reflection across y = x ............> (3,2)
(1,-4) ..............> reflection across y = x ............> (-4,1)
(-3,6) ..............> reflection across y = x ............> (6,-3)
(-5,-7) ..............> reflection across y = x ............> (-7,-5)
Based on the above, all you have to do is get each point of the parallelogram and swap its x and y coordinates to get its image when reflected across y = x
Hope this helps :)