Answer
$$\left\{ {x:x < 12} \right\}$$
Work Step by Step
$$\eqalign{
& \left( { - \infty ,12} \right) \cr
& {\text{Use the interval notations for an Unbounded Interval }} \cr
& \left( {page{\text{ 1152}}} \right) \cr
& \left( { - \infty ,b} \right) = \left\{ {x:x < b} \right\}, \cr
& {\text{Therefore}}{\text{,}} \cr
& \underbrace {\left( { - \infty ,12} \right)}_{\left( { - \infty ,b} \right)} = \underbrace {\left\{ {x:x < 12} \right\}}_{\left\{ {x:x < b} \right\}} \cr
& {\text{The answer is:}} \cr
& \left\{ {x:x < 12} \right\} \cr} $$
