Answer :
4.5*10^3/9*10^7=4.5/9*10^3*10^7=0.5*10^3*10^7. a^b*a^c=a^(b+c), so 10^3*10^7=10^(3+7)=10^10. 0.5*10^10=5*10^9, since we want to write in scientific notation where the leading number is bigger than one and smaller than 10.