Answer
$(a+3)(a^2+1)$.
Work Step by Step
The given polynomial is
$=a^3+3a^2+a+3$
Group the terms.
$=(a^3+3a^2)+(a+3)$
Factor each group.
$=a^2(a+3)+1(a+3)$
Factor out $(a+3)$.
$=(a+3)(a^2+1)$
Hence, the factor of the given polynomial is $(a+3)(a^2+1)$.
