Question 1000286
Solution already posted for the IDENTICAL problem,
<a href="http://www.algebra.com/algebra/homework/word/geometry/Geometry_Word_Problems.faq.question.1000131.html">http://www.algebra.com/algebra/homework/word/geometry/Geometry_Word_Problems.faq.question.1000131.html</a>


That might contain a mistake but the process shown should be clear enough, so study that one, and fix what you can.


---
Using slightly different variables' assignments, here is a better solution:



t for how tall the poster, w for how wide the poster
x for uniform width of the border;
t=w+24
x=6
-
Total area is {{{(t+2x)*(w+2x)}}}
Border Area is  {{{(t+2x)(w+2x)-tw=720}}}

STEPS
{{{tw+2xw+2xt+4x^2-tw=720}}}
{{{2xw+2xt+4x^2-720=0}}}
{{{xw+xt+2x^2-360=0}}}
{{{xw+x(w+24)+2x^2-360=0}}}
{{{xw+xw+2*24+2x^2-360=0}}}
{{{2xw+2*24+2x^2-360=0}}}
{{{xw+24+x^2-360=0}}}
{{{xw=360-24-x^2}}}
{{{w=(360-24-x^2)/x}}}
Substitute for the assigned x=6
{{{w=(360-24-36)/6}}}
Evaluate w,
{{{w=60-4-6}}}
{{{highlight(w=50)}}}------width of poster


{{{t=50+24}}}
{{{highlight(t=74)}}}-------how tall the poster