a square has four equal sides, and we know the perimeter or namely the sum of all those four sides is 1/7 yard, so
s = length of one side.
[tex]s + s + s + s = \cfrac{1}{7}\implies 4s=\cfrac{1}{7}\implies s=\cfrac{~~ \frac{1}{7}~~}{4}\implies s=\cfrac{~~ \frac{1}{7}~~}{\frac{4}{1}} \\\\\\ s=\cfrac{1}{7}\cdot \cfrac{1}{4}\implies s=\cfrac{1}{28}~yd\qquad \stackrel{\textit{how many feet is that?}}{s = \cfrac{1~yd}{28}\cdot \cfrac{3~ft}{yd}}\implies s=\cfrac{3}{28}~ft[/tex]
