Answer
$(-\infty,3]$.
The graph is shown below.
Work Step by Step
The given compound inequality is
$x\leq3$ and $x\lt6$.
First graph then take the intersection of the two inequality.
The graph is shown in the image file.
We can write the compound inequality.
$x\leq3$ as $(-\infty,3]$ and $x\lt6$ as $(-\infty,6)$
The intersection is
$(-\infty,3]\cap(-\infty,6)=(-\infty,3]$.
