Answer:
![\boldsymbol{(-\infty, 3]}](https://img.qammunity.org/2024/formulas/mathematics/high-school/lwapphuoxck06oagvxbxak9x6s116z634x.png)
There's a square bracket for the endpoint "3".
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Work Shown
First we isolate x.

The inequality sign flips when dividing both sides by a negative number.
The inequality
can be thought of as

That would then condense to the interval notation
![\boldsymbol{(-\infty, 3]}](https://img.qammunity.org/2024/formulas/mathematics/high-school/lwapphuoxck06oagvxbxak9x6s116z634x.png)
We use a square bracket for the "3" to include that endpoint.
Either infinity will always get a curved parenthesis.