Tell me what i did wrong and what the answer is i need !

First in my understanding you dont need to convert it  too long 
Just make the 10th power same like exponent of 10's.

if u are comparing two number try to make their 10 ^x factor same

9.98*10^6 kilogram 998*10^4

7.3*10^7 kilogram  73*10^6  this is greater

73*10^6-998*10^4
10^4(7300-998)
10^4(      6302)
6302*10^4 or u can write 6.302*10^7 more