To simplify the expression \(-17 + 5 + 3i - 8\), we need to combine the real parts and the imaginary parts.
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Combine the real parts: \[ -17 + 5 - 8 = -20 \]
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The imaginary part is simply: \[ 3i \]
Putting it all together, we have: \[ -20 + 3i \]
Thus, the expression simplified in the form \(a + bi\) is: \[ \boxed{-20 + 3i} \]