Chapter 9 - Section 9.4 - Imaginary Numbers as Solutions of Quadratic Equations - Exercise Set - Page 655: 9

$-2i\sqrt{7}$

Using the properties of radicals and that $i=\sqrt{-1}$, then the given expression, $ -\sqrt{-28} ,$ is equivalent to \begin{array}{l}\require{cancel} -\sqrt{-1}\cdot\sqrt{28} \\\\= -i\cdot\sqrt{4\cdot7} \\\\= -i\cdot\sqrt{(2)^2\cdot7} \\\\= -i\cdot2\sqrt{7} \\\\= -2i\sqrt{7} .\end{array}