The long way to compute the sum is to expand each number completely, then add:
3.61 * 10^5 = 3.61 * 100,000 = 361,000
6.05 * 10^6 = 6.05 * 1,000,000 = 6,050,000
Then
3.61 * 10^5 + 6.05 * 10^6 = 6,411,000 = 6.411 * 10^6
A shorter way would be to expand the larger of the two numbers until the powers of 10 match up, then add:
6.05 * 10^6 = 60.5 * 10^5
Then
3.61 * 10^5 + 60.5 * 10^5 = (3.61 + 60.5) * 10^5 = 64.11 * 10^5 = 6.411 * 10^6
