Chapter 10 - Exponents and Radicals - 10.8 The Complex Numbers - 10.8 Exercise Set - Page 686: 22

$4-2i\sqrt{15}$

Using $i=\sqrt{-1}$ and the properties of radicals, the given expression, $ 4-\sqrt{-60} ,$ is equivalent to \begin{array}{l}\require{cancel} 4-\sqrt{60}\cdot\sqrt{-1} \\\\= 4-\sqrt{4\cdot15}\cdot i \\\\= 4-\sqrt{(2)^2\cdot15}\cdot i \\\\= 4-2i\sqrt{15} .\end{array}