Answer
$(-\infty,9)$
Work Step by Step
The set of elements both in $(-\infty,9)$ and $(-\infty,12)$ is $(-\infty,9)$ because the first set is all elements less than $9$ and the second set is all elements less than $12$. Thus if something is in both, then it is less than $9$ and $12$, but $12\gt9$, so this is equivalent to something being less than $9$.
