What is the scientific notation of 5,100,000

Your answer should contain no more than one number before the decimal. Simply add the all the numbers except the five and put them behind the decimal. The zeros are insignificant so you don't need to have them behind the decimal. You still need to add them into your problem, you just don't need to write them. So, your answer would be:
5.1 times ten to the sixth or
5.1X10**6