scientific notations are easy once you get the hang of it. basically you're just trying to rewrite a really big or small number in a more easy to understand format. you have to move the decimal point to make a that is above 1 (1 is included) and less than 10, so you just keep moving the decimal point to the right until you get to 7.21 since that is between 1 and 10, but you're not done. now you have to write how far the decimal place moved in this case it moved 4 to the right so you write 7.21x10^-4. if instead your example was 72100 then it moves 4 to the left and you have 7.21x10^4.
