Remember the magnitude (length) of a vector is:
Where are the number of coordinates you have.
In this case we have 3. So the length or
By the way, did you come up with C = (2, 4 , 17) on your own? Or was that a given? Also, is it specific where the points are on the cube? From that picture they all look to be on the same side, but that might not be true.