How do Repetier coordinates setup work I am using Repetier Host and Slic3r Prusa 1.36.2 - (the website says 1.2.9 is the latest version - however I don't believe this is the problem) .  The printer is a Anet A3

With my current setup I am printing mirrored - I can mirror the objects in Repetier before slicing , however then the preview picture is wrong, and worse I have to remember.

I have a Anet A3, the connectors for the motors are polarized and if I swap the X and Y cables when I home the X and Y motors move in the wrong direction.

I believe the problem is the coordinates and I don't fully understand the Repetier coordinates. This is how I believe it is setup (these are Slic3r screen shots ) 
 

Which I will call "Front Left" however my home position is Front Right - ie I believe it should look like this



However these two screen shots are from the slicer - and these settings do not get used.  In the Repetier they use a different system for setup.  I tried these settings shown  in Repetier



Then everything printed in a line along the left most edge. That is the head was moved to the very right , never moved again then the platform just moved 

Can someone explain what settings I should be using for Repetier for a platform setup as I have.  

If I've misunderstood the Slic3r/Repetier interface then you can point that out as well.
I'm not sure exactly sure if you have one problem or two distinctive ones.  However a few notes:

The latest version of Slic3r PE can always be fetched on their GitHub release page.  At the time of writing that would be version 1.38.5, but I concur with you that that is unlikely to be the problem.

Mirrored objects are a clear indication that one of the Cartesian axis is swapped (not that it is translated).  Any of the axis can be swapped, but since the peculiarity of your printer is that the homing point is at the front RIGHT, I would guess the axis that got messed up is the X one.

I have no way to test the following, but if I got the meaning of your first two pictures, then I guess the correct settings in Repetier may probably be these:



If even that fail, check the handedness of the coordinate system of the software generating the model you are trying to slice.  Modelling software like Blender and others are typically right-handed, but many tools for processing those model for on-screen application (movies, videogames) are left-handed.  That means that even if the model display correctly in the native editor, it may be rendered mirrored in another software.