Answer
$3u^2(u^5+4)$
Work Step by Step
Both terms have at least a factor of $u^2$ shared, so that can be factored out. $3$ and $12$ have a shared factors of $3$, so $3$ can also get factored out. That means that $3u^2$ can be factored out to get $3u^2(u^5+4)$, since $u^5(u^2)=u^7$ upon distribution.