Let
w--------> width of the border
we know that
(11-2w)*(13-2w)=80--------> 11*13-22w-26w+4w²=80
143-48w+4w²-80=0
4w²-48w +63=0
using a graph tool------> to resolve the second order equation
see the attached figure
the solution are
w1=1.5
w2-10.5--------> is not solution
the answer is
the width of the border is w=1.5 in
