Length of side s of the square shown below

The answer is:  [A]:  " 3 √2 " .


Solve for "s" ;

      s²  +  s²  =  6²  ;

          2s² = 36 ;

Divide EACH side of the equation by "2" ;

          2s² / 2  =  36 / 2  ; 

to get: 


          s²  = 18 ;

Take the positive square root of EACH side:

         
        √(s²) = √(18)


          s = √(18) = √9 * √2  = 3 √2 ;  which is:  Answer choice:  [A].