You calculate that 319 000 000 m is the answer to a problem. You are asked to write your answer in scientific notation. Which answer is correct?

3.19 raised to the 9th

The scientific notation for 319,000,000 will be 3.19 x 10^8. It is just one of the possible answers, it could be presented in many form. We could express that also as 31.9 x 10^7 or 319 x 10^6. This is just a simple counting of how many numbers are before the period.