Which of the following expresses 393,500 in scientific notation?

Scientific notation is basically a way of saying how many 0's are in this number.

Now keep in mind each number has an invisible decimal at the end:

393,500. (continuing on) So move the decimal to where the 0's are "present"

393.5 x 10 ^ - 3. this says you moved the decimal 3 spaces to the right.

To check move it 3 spaces to the left and you should get your original answer!