To find the scale factor for dilating a square from a side length of 6 units to a side length of 21 units, you can use the following formula for the scale factor:
\[ \text{Scale Factor} = \frac{\text{New Side Length}}{\text{Original Side Length}} \]
In this case:
\[ \text{Scale Factor} = \frac{21}{6} = 3.5 \]
Therefore, the scale factor to transform the square from a side length of 6 units to a side length of 21 units is 3.5.