To solve the expression \((3.4 + \sqrt{-25}) - (\sqrt{-9})\), we first need to identify the square roots of the negative numbers involved.
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Calculate the square roots:
- \(\sqrt{-25} = \sqrt{25} \cdot \sqrt{-1} = 5i\)
- \(\sqrt{-9} = \sqrt{9} \cdot \sqrt{-1} = 3i\)
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Substitute the square roots back into the expression: \[ (3.4 + 5i) - (3i) \]
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Simplify: \[ 3.4 + 5i - 3i = 3.4 + (5i - 3i) = 3.4 + 2i \]
Thus, the result in the form \(a + bi\) is: \[ \boxed{3.4 + 2i} \]