Depends on what you are wanting.
6.4 million is 6,000,000 plus .4 of 1,000,000 (million)
.4 * 1,000,000 = 400,000
So you can write 6.4 million as 6,400,000. In scientific notation:
Also you wrote it already, so what's the problem? Lol j/m.
one million has 6 zero's jsut remember that:
1,000,000 = 1 million
2,000,000 = 2 million
and so on...
say you wanted 1 million 500 thousand:
1,500,000
This is because '500,000' is the 500 thousand part, and the 1 in front is the 1 million.
So 6.4 million would be:
6,400,000
BECAUSE it is 6 million, and 4/10 of a million (400,000)
Also you could write it as: 6.4 x 10^5
Because it is 6.4 multiplied by 10 5 times - there are 5 zero's.
Hope this helps