this means that as the length of a side increases, the diagonal of the octagon increases at a constant amount as well.
Think of a line: y=mx
where y is the diagonal, and x is the side.
now substitute known values: 23/6=m
now that gives equation: y=(23/6)x
now substitute 15 for side length: y=(23/6)(15)=57.5
so the diagonal is 57.5