I will do my best to help! Since I'm not the best at explaining things, I will just show my work. For number four, I wasn't exactly sure what you meant so I just solved it in different ways depending on the different ways I thought you meant. I'm sorry if I didn't end up solving it the way you wanted though. Either way, I really hope I helped you out!
1.
(8.41 X 104) + (9.71 X 104)
874.64 + 1009.84
= 1884.48
2.
(5.11 X 102) - (4.2 X 102)
521.22 - 428.4
= 92.82
3.
(8.2 X 103) + (4.0 X 103)
844.6 + 412
= 1256.6
4. (If the equation was supposed to be "(6.3 X 10^9) - (2.1 X 102)")
(6.3 X 10^9) - (2.1 X 102)
(6.3 x 1000000000) - (2.1 X 102)
6300000000 - 214.2
= 6299999785.8
4. (If the equation was supposed to be "(6.3 X 109) - (2.1 X 102)")
(6.3 X 109) - (2.1 X 102)
686.7 - 214.2
= 472.5
