Answer
$(-\infty,5)$
See the figure below.
Work Step by Step
Distribute $-3$ and then add $3$ to both sides to obtain
$$
- 3(1-x )\lt 12\Longrightarrow -3+3x \lt 12\\
\Longrightarrow -3+3x+3 \lt 12+3 \Longrightarrow 3x\lt 15
.$$
Dividing both sides by $3$, we get
$$
3x\lt 15 \Longrightarrow x \lt 5
.$$
In terms of intervals, $x\in (-\infty,5)$.
To graph, plot a circle at $x=5$ then shade the region to its left.
See the figure below.
