To solve the equation \(x^2 + 9 = 0\), we can first isolate \(x^2\):
\[ x^2 = -9 \]
Next, we take the square root of both sides. Remember that the square root of a negative number will involve the imaginary unit \(i\):
\[ x = \pm \sqrt{-9} = \pm \sqrt{9}i = \pm 3i \]
Thus, the solutions to the equation \(x^2 + 9 = 0\) are:
\[ x = 3i \quad \text{and} \quad x = -3i \]
Looking at the options provided, the correct answer is:
\[ {3i, -3i} \]