Question 926243
w and L the dimensions;
L=w+2 and 2w+2L=20.
Simple substitution


{{{2w+2(w+2)=20}}}
{{{w+w+2=10}}}
{{{2w+2=10}}}
{{{w+1=5}}}
{{{highlight(w=4)}}}
and the L=4+2