In scientific notation, you need a number from 1 to less than 10 multiplied by a power of 10.
To turn 384,400 into a number from 1 to under 10, place a decimal point after the 3.
Now you have a number from 1 to less than 10. 3.844
Now count the number of places the decimal point moved left from its original position. The decimal point was here: 384,400. after the last zero. The decimal point moved 5 places to the left to be placed after the 3. Since the decimal point moved 5 places left, the exponent of the 0 is 5.