First we have to find length of 1 side of the square. We can use formula for area of the square. A=a*a ==> 
where a is length of the side.
From this formula we can find
a=
Now we just have to multiply it by 4, because there are 4 sides.
81*4=324 - its the answer
From this formula we can find
a=
Now we just have to multiply it by 4, because there are 4 sides.
81*4=324 - its the answer