[tex]\text{The scientific notation}\ a\cdot10^k,\ \text{where}\ 1\leq a<10\ \text{and}\ k\in\mathbb{Z}.\\\\(9\times10^7)(7\times10^9)=(9\times7)(10^7\times10^9)=63\times10^{7+9}=63\times10^{16}\\\\=6.3\times10\times10^{16}=\boxed{6.3\times10^{17}}\\\\\text{used}\ a^n\cdot a^m=a^{n+m}[/tex]
