3.
$\lim(-n^2+5n-1)$
$=\lim n^2\Big(-1+\dfrac{5}{n}-\dfrac{1}{n^2}\Big)$
$=-\infty$
4.
$\lim(\sqrt{n^2-2n}-n)$
$=\lim \dfrac{n^2-2n-n^2}{\sqrt{n^2-2n}+n}$
$=\lim \dfrac{-2n}{\sqrt{n^2\Big( 1-\dfrac{2}{n}\Big) }+n}$
$=\lim \dfrac{-2}{ \sqrt{1-\dfrac{2}{n}}+1}$
$=\dfrac{-2}{1+1}$
$=-1$