So... Dont laugh... I have drawn an image/shape with the brush (stroke no fill) and I want to select it, copy it and reflect it horizpntally so I have an exact mirror copy (its an eye so im going for symmetry here) It is on its own layer with the rest of the line art.
Drag out a guide to the center of the face. Drag out a rectangular selection from that guide, enclosing the eyes. Free Transform a copy (Alt+Control+T), setting the reference point to middle left, and horizontal scaling to -100%.