What is the standard form of the number 39000?

The Standard form of the number 39000 is... 3.9 X 104. In standard form, the first significant figure is in the units column (in this case the 3), and the index (the little number) is the number of columns this first significant figure is away from the units column in the actual number. So in 39000, the 3 is four columns away from the unit column, which is why it is 104.

Hope that cleared it up. :)