Question 884808
What do you know?
Let x = side length of square;  {{{4x}}} is perimeter, {{{x^2}}} is area
Let s = side length of equilateral triangle; {{{3s}}} is perimeter


What does the problem description tell you?
{{{x=-3+s}}};  4x is perimeter for square;  3s is perimeter for traingle;  {{{4x=3s}}}.


Solve Equations:
System is {{{x=s-3}}} and {{{4x=3s}}}.
{{{4(s-3)=3s}}}
{{{4s-12=3s}}}
{{{highlight(s=12)}}}
-
{{{x=-3+s}}}
{{{x=-3+12}}}
{{{highlight(x=9)}}}


Area of the square is {{{highlight(highlight(x*s=9*12))}}}.