#### Answer

$\sqrt{10}
\text{ units}$

#### Work Step by Step

Using $|a+bi|=\sqrt{a^2+b^2},$ then the given expression, $
|-3-i|
,$ is equal to
\begin{array}{l}\require{cancel}
\sqrt{(-3)^2+(-1)^2}
\\\\=
\sqrt{9+1}
\\\\=
\sqrt{10}
\text{ units}
.\end{array}