Question 998674
The entire poster is the print plus the border.  First assign variables to everything.


{{{system(h=heightPrint,w=widthPrint,u=widthBorder,A=areaPoster)}}}


These are the known values:
{{{system(u=3,A=680)}}}.
Description gives {{{h=2w}}}.


The entire poster includes the border and is the area A.
{{{hihlight_green((h+2u)(w+2u)=A)}}}


{{{hw+2uw+2hw+4u^2-A=0}}}
{{{2w*w+2uw+2*2w*w+4u^2-A=0}}}
{{{6w^2+2uw+4u^2-A=0}}}------quadratic equation in unknown, w.  Remember, w is the width of the PRINT.


Keep going like this in all symbols if you want, but now is a good place to substitute the known values and simplify.
{{{6w^2+2*3*w+4*9-680=0}}}
{{{6w^2+6w-644=0}}}
{{{highlight_green(3w^2+3w-322=0)}}}

Discriminant, {{{9+4*3*322=9+12*322=3873}}}
{{{3873=3*1291}}}, being two PRIME factors, so just stay with  3873 as discriminant.


{{{highlight(w=(-3+sqrt(3873))/(2*3))}}}, the width of the PRINT (not the poster).  You can compute this and then finish finding the height, h.