To find the length of the segment \( \overline{RS} \), we will use the distance formula between the points \( R(-3, 3) \) and \( S(-3, -9) \).
The distance formula is:
\[ d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2} \]
In our case, \( R(x_1, y_1) = (-3, 3) \) and \( S(x_2, y_2) = (-3, -9) \).
Substituting the coordinates into the distance formula, we have:
\[ d = \sqrt{((-3) - (-3))^2 + ((-9) - (3))^2} \]
Calculating the differences:
- The \( x \)-coordinates: \( -3 - (-3) = 0 \)
- The \( y \)-coordinates: \( -9 - 3 = -12 \)
Now substituting back into the formula:
\[ d = \sqrt{(0)^2 + (-12)^2} \]
Calculating the squares:
\[ d = \sqrt{0 + 144} = \sqrt{144} = 12 \]
Thus, the length of \( \overline{RS} \) is \( \boxed{12} \) units.