From the first picture we know "The ISS's orbital distance from Earth's center (to) is the sum of its altitude above Earth's surface (a) and the radius of Earth itself (re), as modeled below'

So, ro = a + re = 0.42 * 10^6 + 6.37 * 10^6 = 6.79* 10^6

You can see we changed 4.2 * 10^5 = 0.42 * 10^6 

Hope this helps

Elain,

How do I know to change 4.2*10^5 to 0.42*10^6?

Because it is easier to add numbers that are both multiplied by the same power of 10, just like it is easier to add fractions that have the same denominator.

Thank you, Janice Holz.

You're welcome ☺