How can I find the reflection of a point in a perpendicular line

From the image below I know the positions A and B. How can I find positions C and D and the reflection (Er) of an object E in the line CD? Please note that I'm trying to achieve this in 2D.

 

can someone please help me out

 

Your description isn't really exact. Is CD exactly through the middle of AB?

Assuming so, here's roughly how I'd attempt it:
find the origin, where they cross - assuming the middle that'd be O=A+(B-A)/2
find the perpendicular vector - one would be P={(b.y-a.y),(a.x-b.x)}
find the direction from E to O -> EO=O-E
use Vector2.Reflect to reflect EO to OEr -> OEr = Vector2.Reflect(EO, P)
add OEr to O to get Er=OEr + O

There may be some simpler way or some errors in this (normalizing missing probably), but it's one approach.