An ant walks around on the coordinate plane. It moves from the origin to $(3,4)$, then to $(-9, 9)$, then back to the origin. How many units did it walk? Express your answer as a decimal rounded to the nearest tenth.
1. **Calculate the distance from the origin \((0,0)\) to the point \((3,4)\):**

   The distance formula between two points \((x_1, y_1)\) and \((x_2, y_2)\) is given by:
   \[
   d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}
   \]
   Applying this to the points \((0,0)\) and \((3,4)\):
   \[
   d_1 = \sqrt{(3 - 0)^2 + (4 - 0)^2} = \sqrt{3^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5
   \]

2. **Calculate the distance from \((3,4)\) to \((-9,9)\):**

   Using the distance formula again:
   \[
   d_2 = \sqrt{(-9 - 3)^2 + (9 - 4)^2} = \sqrt{(-12)^2 + 5^2} = \sqrt{144 + 25} = \sqrt{169} = 13
   \]

3. **Calculate the distance from \((-9,9)\) back to the origin \((0,0)\):**

   Using the distance formula once more:
   \[
   d_3 = \sqrt{(0 - (-9))^2 + (0 - 9)^2} = \sqrt{9^2 + 9^2} = \sqrt{81 + 81} = \sqrt{162} \approx 12.7
   \]

4. **Add up all the distances:**

   \[
   \text{Total distance} = d_1 + d_2 + d_3 = 5 + 13 + 12.7 = 30.7
   \]

The final answer is $\boxed{30.7}$