what is the length of each side of a square that has the same area as a rectangle that is 12mm long and 18mm wide?

The area of the rectangle = 12mm X 18mm = 216mm^2

And the area os a square = side^2

Thus

side^2  = 216    take the sq root of both sides

side = sqrt 216  = sqrt (36 * 6) = 6*sqrt(6)   ......and that's in mm