Answer
$n=0$ or $n=-5$
Work Step by Step
$ n(n-3)=2n(n+1)\qquad$..add $-2n(n+1)$ to both sides.
$ n(n-3)-2n(n+1)=0\qquad$..factor out $n$.
$n(n-3-2(n+1))=0$
$n(n-3-2n-2)=0$
$ n(-n-5)=0\qquad$...apply the principle of zero products.
$n=0$
or
$-n-5=0$
$n=-5$
