Answer
$x\leq-1$
Work Step by Step
The $''(-\infty,...'' $indicates that this interval contains numbers that can be negative and unbounded in magnitude (so there is no left endpoint).
At the same time, they are less or equal to $-1$,
since the bracket on the right indicates that the right endpoint is included..
In less words, these are numbers that are less than or equal to $-1$.
For a number x from this interval, we can write
$x\leq-1$
