Let x represent the length of each of the two parallel sides of fencing and y represent the length of remaining third side.
There are 680 ft of fencing available to fence parking, then x+x+y=680, 2x+y=680 and y=680-2x - this is the expression for the length of the remaining side to be fenced in terms of x.
Obviously that x>0 and y>0 (because x and y represent lengths). Then 680-2x>0, -2x>-680,2x<680,x<340.
You have that 0<x<340.