This looks like homework, so here's just a hint:
As the comments point out, there is no reason to think that it can only break at the inch marks. A more realistic model would be that the break point is uniformly distributed from 0 to 15. In this case, there are two ways you can get a piece at least 12 inches. Let $p$ be the point where the break occurs:
$$P(p<3 \;\;\text{or} \;\;p>12)$$
Can you take it from there?