mks stands for meters, kilograms, and seconds. Most formulas that you use in physics require that the numbers be in these units. For example, distance = velocity * time. If your velocity were 20 meters/second and your time was 2 hours, it would not be correct to say that distance = 20 m /s * 2 hours = 40. (Actually, you could do it this way if you were satisfied with a unit that no one used. It would be 40 meter hours/second. But try telling someone that the distance from their house to yours is 40 meter hours/second, and you'll gain an appreciation for mks). But if your time was in seconds then you would have distance = 20 m/s * 7200s= 14400 m. Now if you want, you can stray from mks and express the answer as 14.4 km, which in everyday talk is the best way to say it. But as long as you remain in mks while working the problem, you'll avoid lots of mistakes.
Look at the units in your problem:
G: N m^2/kg^2 : you've got meters and kilograms
Mass: kg
Radius: km
Everything looks good except for radius. It's expressed in km, and the formula wants meters. So you must convert it to meters first by multiplying it by 1000.
When you get an answer, your answer will be in meters, but it would be easier to comprehend if it were in kilometers. So you would want to divide it by 1000.
In your problem, notice the units after G. There's a "m" in there and a "kg".