This is the concept of linear proportionality, the linear scale factor of the polygon given will be:
[perimeter of larger polygon]/[perimeter of smaller polygon]=[length of larger polygon]/[length of smaller polygon]
=28/20
=7/5
thus the length of the larger polygon will be given by:
7/5=x/4
thus;
7/5*4
=5.6
thus the length 5.6