Tracy dilates the square below. The perimeter of the dilated square is 84 millimeters.

Let's denote the side length of the dilated square as x.

Since Tracy dilated the square by a factor of x/6, the perimeter of the dilated square can be calculated as 4 * x = 84.

Therefore, x = 84 / 4 = 21 mm.

So, the side length of the dilated square is 21 mm.