Question 1185239
The dimensions of any side of the given square is 9 units.  Perimeter of this given square is 36 units.


Equilateral triangle with that same 36 unit perimeter, each side of this triangle is 12 units.


Area for this triangle:
This can be cut into two right triangles with hypotenuse 12, short leg 6, and longer leg a, so that   {{{a^2+6^2=12^2}}}.  You can solve for a.
-
{{{sqrt(9*12)}}}
{{{sqrt(3*3*3*2*2)}}}
{{{highlight_green(a=6*sqrt(3))}}}



The equilateral triangle's area:
{{{2(1/2)12*a}}}
or
{{{highlight_green(12a)}}}