Chapter 7 - Exponents and Exponential Functions - 7-3 Multipying Powers with the Same Base - Practice and Problem-Solving Exercises - Page 430: 53

$2.1 \times 10^{-5}$

$a^m \cdot a^n = a^{m+n}$ Multiply the corresponding parts and use the rule above when necessary to obtain: $=(0.7\cdot 0.3) \times (10^{-12} \cdot 10^{8}) \\=0.21 \times 10^{-12+8} \\=0.21 \times 10^{-4} \\=(2.1)(10^{-1}) \times 10^{-4} \\=2.1 \times 10^{-1+(-4)} \\=2.1 \times 10^{-5}$