In scientific notation, the first number must be at least 1 but less than 10. Moving the decimal gives 8.4, and the number of decimal moves gives the power -4. Scientific notation is written as a number from 1 to less than 10 multiplied by a power of 10. Large numbers use positive powers, while small decimals use negative powers. The number of decimal places moved becomes the exponent, so keep track of both the direction and the count. The correct answer is 8.4 × 10^-4, which matches the calculation or reasoning shown.