Scientific notation is a representation of a large number in a short form. The answer, 1*10^12 (read as ten to the twelfth power) essentially says that the number when expanded is 1 with 12 zeros appended to it. If you did a number like 7 trillion you would have 7 * 10^12.
Same thing can be applied for 1,000,000 (one million).
1 * 10^6 because one million has 6 zeros appended to it.
