Chapter 7 - Section 7.7 - Complex Numbers - Exercise Set - Page 464: 108

$5-4i$

Using $i=\sqrt{-1}$ and the properties of radicals, the given expression, $ 5-\sqrt{-16} ,$ is equivalent to \begin{array}{l}\require{cancel} 5-\sqrt{-1}\cdot\sqrt{16} \\\\= 5-i\sqrt{(4)^2} \\\\= 5-i\cdot4 \\\\= 5-4i .\end{array}