Electromagnetic transverse wave question

[Blitzy89]

Hello. I'm having some difficulty with a question from Optics (4th edition, Hecht).
http://milicioner.hit.bg/newphysicsquestion.GIF
I included the answer in the same image (it is at the end of the book), but I can't understand why (-E/(rsqr2))i must be negative.
Thank you

 

[vela]

What is the vector pointing along the direction of propagation? Then remember E has to be perpendicular to it because light is a transverse wave.