## Precalculus (6th Edition) Blitzer

$-3i$
RECALL: (1) $\sqrt{-1}=i$ (2) For any real number $a\gt0$, $\sqrt{-a} = i\sqrt{a}$. Use rule (2) above to obtain: $=i\sqrt{81}-i\sqrt{144} \\=i\sqrt{9^2} - i\sqrt{12^2} \\=i(9) - i(12) \\=9i-12i \\=(9-12)i \\=-3i$