You haven't provided the parallelogram , therefore, I cannot give an exact answer. However, I will help you with the concept.
<u>When reflecting a point across the line y = x:</u> We swap the values of x and y while keeping their sign the same.
<u>This means that:</u> (x,y) ............> reflection across y = x ..........> (y,x)
<u>Examples:</u> (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