Answer
$\{x\ |\ \ x \leq 3.5 \ \}$
Work Step by Step
Recall the general rules for writing intervals:
An interval may be annotated as
$[a,b],\ (a,b],\ [a,b),\ (a,b),\ (-\infty,b],\ (-\infty,b),\ [a,\infty),\ (a,\infty).$
It contains numbers between the left and right borders. The inequality signs depend on whether a border is included in the set or not.
The bracket "[", or "]" means "border included" and the sign is "$\leq $".
The parenthesis "(" or ")" means "border excluded" and the sign is "$\lt $".
$\pm\infty $ implies "no border", so it is always accompanied by a parenthesis.
For example
$[a,b)=\{x\ |\ a\leq x\lt b\ \}$
$(-6,\infty)=\{x\ |\ x\gt-6\ \}$
$(-\infty,3]=\{x\ |\ \ x\leq 3\ \}$
---
Here, the left border is unbounded.
and the right border is included.
We write this as
$-\infty \lt x \leq 3.5$
or, simply as: $\qquad x \leq 3.5$
$ (-\infty,3.5]$ = $\{x\ |\ \ x \leq 3.5 \ \}$
