Chapter 9 - Section 9.4 - Imaginary Numbers as Solutions of Quadratic Equations - Exercise Set - Page 655: 2

$7i$

Using the properties of radicals and that $i=\sqrt{-1}$, then the given expression, $ \sqrt{-49} ,$ is equivalent to \begin{array}{l}\require{cancel} \sqrt{-1}\cdot\sqrt{49} \\\\= i\cdot\sqrt{(7)^2} \\\\= i\cdot7 \\\\= 7i .\end{array}