1. Move the decimal point: Start with 390,000.0 and move the decimal point to the left until you have a number between 1 and 10. In this case, you'll move it five places to the left to get 3.9.

2. Count the decimal places: You moved the decimal five places to the left.

3. Write in scientific notation: The result is 3.9 x 10⁵

Therefore, 390,000 in scientific notation is 3.9 x 10⁵.