Answer
so I need to know the coordinates for me to tell you the answer but I think I can still help you by explaining.
In order for E to become E' the rule for reflection over y=x is (y, x) so you basically switch the x and the Y to have E'. so for you to be able find out E, you need to witch the x and the y.
for example:
if E' was (-2, 3)
E in the pre image would be (3, -2)
hope this helps :)
