In the problem, you are told to add the numbers, so you will need to follow the rules on page 28 of the textbook for adding with significant figures. You will need to expand the number given in scientific notation before adding. Once you've added, you must use precision to determine where to round.
When you look back to the precision, the 2.30040 x 106 is significant to the last 0, which, when expanded to 2,300,400 that 0 is in the tens column. That is the least precise number. You're not counting significant figures here, but precision. The 7,345.6 is more precise than 2,300,400. Therefore, the answer must be rounded to the tens column.
Tags: Chemistry, Significant Figures