When subtracting scientific notation, you first need to make the exponents the same. The standard is to make the smaller exponent the same as the higher one.
Let's take a look at your given:
[tex]7.09 x 10^{5}[/tex]
[tex]6.10 x 10^{4}[/tex] This is smaller.
Every time you add to the exponent, you need to move the decimal. So to make it the same, you need to add 1 to the 4 to get 5. Then move the decimal to the left. Your new notation would be:
[tex].610 x 10^{5}[/tex]
Then you just line up the decimal as you would before subtracting and just copy the base and exponent.
[tex]7.09 x 10^{5}[/tex]
- [tex].61 x 10^{5}[/tex]
=
[tex]6.48 x 10^{5}[/tex]
