To find the scale factor between two circles, you can divide the diameter of the final circle by the diameter of the original circle.
Given:
- Diameter of the original circle = 3 cm
- Diameter of the final circle = 21 cm
Scale Factor = \(\frac{\text{Diameter of final circle}}{\text{Diameter of original circle}} = \frac{21 \text{ cm}}{3 \text{ cm}} = 7\)
So, the scale factor is \(7\).