Question

A living room wall is 11 ft long. How far from the corner would you have to the edge of a 2ft 6 inch shelf for it to be centered on the wall

Answer 1

It would just be 5'5" + 1'3" = 6'9".
Take the length of the wall & then subtract the length of the shelf. And then divide by 2. That tells you how far to measure in from the left for the left edge of the shelf. Hope that helped a little.