To solve the equation \( x^2 + 9 = 0 \), we can start by rearranging the equation:
\[ x^2 = -9 \]
Next, taking the square root of both sides, we get:
\[ x = \pm \sqrt{-9} \]
Since \(\sqrt{-9} = \sqrt{9} \cdot \sqrt{-1} = 3i\), we find:
\[ x = \pm 3i \]
This means the solutions to the equation are:
\[ x = 3i \quad \text{and} \quad x = -3i \]
Therefore, the correct response that makes the equation true is:
\({3i, -3i}\)