Answer
$-5\sqrt 3$
Work Step by Step
$\sqrt -5\times\sqrt -15=i\sqrt 5\times i\sqrt 15=i^{2}\times\sqrt (5\times15)=-1\times\sqrt 75=-\sqrt (25\times3)=-(\sqrt 25\times\sqrt 3)=-(5\sqrt 3)=-5\sqrt 3$
We know that $\sqrt -1=i$, because $i^{2}=-1$.