Here's how to get there:
1. Move the decimal point to the left until you have a number between 1 and 10. In this case, you move it four places to the left, leaving you with 2.1.
2. Count the number of places you moved the decimal. This number becomes your exponent. Since you moved it four places, the exponent is 4.
3. Combine the number and the exponent. This gives you 2.1 x 104.
