Answer
$x>3$
Work Step by Step
The "(3..." indicates that numbers in this interval are greater than 3 (but never equal to 3, since the left endpoint is not included in the interval).
The "...$\infty$)" indicates that these (positive) numbers can be unbounded in magnitude
so there is no right endpoint.
In brief, this interval contains all numbers greater than 3.
For a number x from this interval, we can write
$x>3$
