Question 996598
Assign variables to everything.
{{{system(L=25,w=30,A=504,x=unknownBorderWidth)}}}
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A is for the area of the border to surround the rectangular region.


Total area minus given rectangular region area is the border area;
{{{highlight_green((L+2x)(w+2x)-Lw=A)}}}
The rest should be easy; the unknown is x.