To simplify the expression \( (9 + \sqrt{-4}) + \left(-9 + \sqrt{-16}\right) \), we can first rewrite the square roots of the negative numbers in terms of \( i \).
Recall that:
- \(\sqrt{-4} = \sqrt{4} \cdot \sqrt{-1} = 2i\)
- \(\sqrt{-16} = \sqrt{16} \cdot \sqrt{-1} = 4i\)
Now, substituting these values into the expression:
\[ (9 + 2i) + (-9 + 4i) \]
Now we can simplify this expression:
\[ 9 - 9 + 2i + 4i = 0 + 6i = 6i \]
So the simplified result is:
\[ \boxed{6i} \]