1. Move the decimal point: Move the decimal point to the right until you have a number between 1 and 10. In this case, you'll move it four places to the right:

0.00039 -> 3.9

2. Count the moves: You moved the decimal point four places to the right. This means your exponent will be -4 (negative because you moved it right).

3. Combine the number and exponent:

3.9 x 10⁻⁴

Therefore, 0.00039 in scientific notation is 3.9 x 10⁻⁴