You can
put this solution on YOUR website!lets call the width x and the length y=x+25 of the poster......the cardboard area minus the poster area would give us the area of the border(which is 1350).
the cardboard area would be (x+5)(x+25+5)=(x+5)(x+30)=
the area of the poster is x(x+25)=

:
so

=

:

cm in width
:

cm in length