The question is telling the ratio of radius:diameter is 1:2. This means that to get the diameter from the radius, you must multiply by 2 (and divide the diameter by 2 to find the radius).
To find the speed with given distance and time you just have to divide the distance by time. In this situation it's 396 miles in 6 hours, so it's 396 / 6 = 66 miles in 1 hour. So the speed is 66 mph.
