Answer
$a_9=9k+32$
Work Step by Step
To find $a_9$, use the formula $a_n=a_1+d(n-1)$ to obtain:
\begin{align*}
a_9&=a_1+d(n-1)\\
a_9&=k+(k+4)(9-1)\\
a_9&=k+(k+4)(8)\\
a_9&=k+8k+32\\
a_9&=9k+32
\end{align*}
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