I bought a mirror 9ft *4ft and image is alright till 10 ft away from the mirror after that it's distorting the image and thickening it.How can I solve this problem?
Because the surface you affixed it to is not flat
A mirror that large can't rely on its own stiffness to stay straight. It is relying on the underlying surface. Glass can bend a little bit. Your surface is bending, so the mirror is bending, so you have a funhouse mirror.