Answer
$z^2+z+4$
Work Step by Step
$\begin{aligned}(z^2-5z+8)+(6z-4)&=z^2-5z+8+6z-4\text{ [Remove parentheses]}\\&=z^2+(-5z+6z)+(8-4) \text{[Group like terms]}\\&=z^2+z+4\end{aligned}$
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