It would be
3.5 × 10⁻³ kg.
The decimal point must be moved 3 places to the right in order to have it behind the first non-zero digit; this gives us the exponent of 3, and since we are moving the decimal to the right, it is a negative exponent.
You have to say what or where point B and C is for me to answer it
I need more details in order to answer your question
Are you rounding to the nearest 100’s place?
