x2 + y2 = r2 is the equation for a circle. So for your problem, take the square root of 2250000 to get the radius, in this case, 1500. I am assuming km is the units.
if the orbit of the other is 200km farther out, the radius is 200km greater, so it's radius is 1700km.
you then figure out the distance each travels in the orbit (circumference) c=2rpi,
first satellite c=2(1500)(3.14)=9420km
second satellite c=2(1700)(3.14)=10676km
just subtract the distance the first one travels from the distance the second one travels to get your answer.
If you didn't have the radius of the first satellite's orbit, you could set up an equation as follows: 2(r+200)pi -2rpi =x circumference of larger - circumference of smaller = distance larger travels farther
which reduces to 2rpi + 400pi -2rpi = x
which reduces to 400pi = x
your answer would be the same either way.
hope this helps!