When you need to round a number. First you need to look at the placement number that the question is asking to round to the nearest. In this case it's the nearest hundred.
3,130,649 the 6 is in the hundreds place. Now look to the number to the right of that number 3,130,649 that's the number that tells you if you will round up or down. If the number is 5 or more you round up, if it's less than 5 you round down.
In this case the 4 is less than 5 so you round down.
So your answer would be 3,130,600
Hope this helps. :)