Question 210414
Let the width of the poster = 
{{{w}}} cm
The height then =  
{{{w + 25}}} cm
given:
The area of the border is {{{1350}}} cm2
The width of the particle board will =
{{{w + 5 + 5 = w + 10}}} cm
The height of the particle board will =
{{{w + 25 + 5 + 5 = w + 35}}} cm
The area of the particle board =
{{{(w + 10)(w + 35)}}} cm2
The area of the poster = 
{{{(w + 10)(w + 35) - 1350}}} cm2
The area of the poster is also
{{{w*(w + 25)}}}
{{{w*(w + 25) = (w + 10)(w + 35) - 1350}}} cm2
{{{w^2 + 25w = w^2 + 45w + 350 - 1350}}} 
{{{25w = 45w - 1000}}} 
{{{20w = 1000}}} 
{{{w = 50}}} cm
{{{w + 25 = 75}}} cm
The dimensions of the poster are 50 cm x 75 cm
check:
{{{w*(w + 25) = (w + 10)(w + 35) - 1350}}} cm2
{{{50*(50 + 25) = (50 + 10)(50 + 35) - 1350}}} 
{{{50*75 = 60*85 - 1350}}}
{{{3750 = 5100 - 1350}}}
{{{3750 = 3750}}}
OK