#### Answer

fill the blank with
$a-bi$

#### Work Step by Step

See: Properties of Roots of Polynomial Equations (page 382)
1. If a polynomial equation....
2. If $a+bi$ is a root of a polynomial equation with real coefficients $(b\neq 0)$, then the imaginary number $a-bi$ is also a root.
Imaginary roots, if they exist, occur in conjugate pairs.
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fill the blank with
$a-bi$