You're going to be writing a small length in terms of a really long one. That means the answer is going to be really small. You've chosen to express 50 mm in km, and that's 50 millimeters or 50 thousandths of a meter in terms of kilometers or thousands of meters. To convert millimeters to meters, you'll have to divide by 1,000. And to convert meters to kilometers, you'll have to divide by 1,000. So to convert millimeters to kilometers, you'll have to divide by 1,000,000 (a million), or 106. 50 mm / 106 = 50 x 10-6 km or 5 x 10-5 kilometers, or 0.00005 kilometers.