Answer:
960
Explanation:
We first simplify both square roots:\begin{align*}
\sqrt{128} &= \sqrt{64\cdot 2} = \sqrt{64}\cdot \sqrt{2} = 8\sqrt{2},\\
\sqrt{50} &= \sqrt{25\cdot 2} = \sqrt{25} \cdot \sqrt{2} = 5\sqrt{2}.
\end{align*}Then, we have\begin{align*}
\left(-3\sqrt{128}\right)\left(-4\sqrt{50}\right) &= \left(-3\cdot 8\sqrt{2}\right)\left(-4\cdot 5\sqrt{2}\right)\\
&=\left(-24\sqrt{2}\right)\left(-20\sqrt{2}\right)\\
&=(-24)(-20)\left(\sqrt{2}\right)^2\\
&=480(2)\\
&=\boxed{960}.
\end{align*}