Answer
$$\left(a-2\right)\left(a^2+3\right)$$
Work Step by Step
Factoring the expression completely, we find:
$$ 3\left(a-2\right)+a^2\left(a-2\right) \\ \text {Factor a-2 out of the entire expression } \\ \left(a-2\right)\left(a^2+3\right)$$