To find the scale factor of a dilation, we first find two corresponding sides. For example, the top edge of the black shape corresponds with the top edge of the purple shape. The top edge of the black shape measures 15 units while the top edge of the purple shape measures 3 units
Now, we divide the length taken from the figure by the length taken from the original shape. In this case, we divide the length of 3 by the length of 15.