Question

Have I done question c correctly?

Answer 1

I believe so , I see no mistake! (I am no like genius, so I might be wrong)

Answer 2

No, your answer is not correct. Your answer would have been correct if you were converting tonnes to kilograms.
To fix your mistake, simply multiply 7820000 by 1000 instead of dividing
In that case, you would get 7820000000 or 7.82 x 10^9