&7.3 \times 10^5 \times 6.8 \times 10^4 && \text{Apply the rule } \boxed{(A \times 10^m) \times (B \times 10^n)=(A \times B) \times (10^{m+n})}.\\&(7.3 \times 10^5) \times (6.8 \times 10^4)=(7.3 \times 6.8) \times (10^{{\color{red}5+4}})\\&(7.3 \times 6.8) \times (10^{{\color{red}5+4}})=({\color{red}49.64}) \times (10^{{\color{red}9}})\\&({\color{red}49.64}) \times (10^{{\color{red}9}})=49.64 \times 10^9 && \text{Write the answer in scientific notation.}\\&49.64 \times 10^9=(4.964 \times 10^{{\color{red}1}}) \times 10^9 && \text{Apply the law of exponents }-\text{ add the exponents of the power.}\\&49.64 \times 10^9=4.964 \times 10^{{\color{red}10}}\\&\boxed{(7.3 \times 10^5) \times (6.8 \times 10^4)=4.964 \times 10^{10}}
{}