Answer
$3(x^2-4x+5)$
Work Step by Step
To factor the second-degree polynomial $3x^2 - 12x + 15=3(x^2-4x+5)$, we have to find integers whose product is $5$ and whose sum is $-4$. But there are no such factors. Thus the polynomial $x^2-4x+5$ is prime and hence we have
$$
3x^2 - 12x + 15=3(x^2-4x+5)
.$$
