sqrt{-25}*sqrt{-4}
Note that the familiar property, $\displaystyle \sqrt a\sqrt b = \sqrt{ab}$ applies only to nonnegative $\displaystyle a$ and $\displaystyle b$. Instead, we have
$\displaystyle \sqrt{-25}\sqrt{-4}$
$\displaystyle =\left(i\sqrt{25}\right)\left(i\sqrt4\right)$
$\displaystyle =(5i)(2i) = 10i^2$.
Now, what is $\displaystyle i^2$? Simplify.