Question 865207
Not too clear.  What inner edge?  The garden is square, and so it has an edge.  Do you mean that the entire two acres is the garden plus the pathway?  If yes then try this:


Total area is 2 acres, and side length of this square is {{{sqrt(2)}}} acres.  
Let w = width of the pathway around the garden part.
Actual garden part is {{{(sqrt(2)-2w)^2}}}.
Given that area of pathway is {{{(1/8)}}} acre, you have 
-------- {{{highlight_green(sqrt(2)=(sqrt(2)-2w)^2+(1/8))}}}
and you want to solve for w.  From this, just several algebra steps.