Suppose the polygons have 3 sides, [tex]s_1,s_2,s_3[/tex]. In the smaller polygon, the corresponding sides would have lengths [tex]\dfrac25s_1,\dfrac25s_2,\dfrac25s_3[/tex].
So if the perimeter of the larger polygon is
[tex]s_1+s_2+s_3=85[/tex]
then the perimeter of the smaller polygon would be
[tex]\dfrac25(s_1+s_2+s_3)=\dfrac25\cdot85[/tex]
so its perimeter would be 34 meters.