## Elementary Algebra

$5i\sqrt 3$
Recall, $i=\sqrt{-1}$. Thus, we obtain: $\sqrt {-75}$ $=\sqrt {-1\times3\times25}$ $=\sqrt {-1}\times \sqrt 3\times \sqrt {25}$ $=i\times \sqrt 3\times \sqrt {5^{2}}$ $=i\times \sqrt 3\times 5$ $=5i\sqrt 3$