We have been given that wall is
feet long.
Let us convert it in improper fraction,

Since we know that 1 feet equals to 12 inches. Now we will convert length of wall in inches.
We have been given that mirrors are hung 3 feet apart that will be,
Now we will add width of both mirrors and distance between both mirrors.

Now we will subtract 96 from 135 and divide our difference by 2 to find distance between the edge of wall and the edge of each mirror.

Therefore, distance between the edge of the wall and the edge of each mirror will be 19.5 inches.