Answer:
3.921 * 10^-3
Explanation:
First, move the decimal place until you have a number between 1 and 10. If you keep moving the decimal point to the right in 0.003921 you will get 3.921 after moving it 3 times to the right.
since you moved it three times to the right three times, the exponent will be -3.
Therefore, the solution is 3.921*10^-3