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