First find the area of the rectangle:
now since all the sides of a sqr are equal and area of sqr=side*side:
side of sqr=sqr root of 36=6
therefore , perimeter of sqr=6*4=24m
diagonal will turn the sqr into 2 congruent triangles
, of which the 2 sides are 6m and 6m.
therefore the 3rd side can be found using pythagoras theorem:
3rd side=sqr root of 72
hope you understand and this helped...