Newton's law of gravitational

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