Answer
$$1.53 \times 10^{-11}$$
Work Step by Step
\begin{align}
(5.1 \times 10^{-8})(3 \times 10^{-4}) & = (5.1 \times 3)(10^{-8} \times 10^{-4})\\ &= 15.3 \times 10^{-8-4} \\ &=15.3 \times 10^{-12} \\ &= 1.53 \times 10^{-11}
\end{align}
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