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**eXist** Remember the magnitude (length) of a vector is:

$\displaystyle l = \sqrt{x_1^2 + x_2^2 + ... + x_n^2} $

Where $\displaystyle x_1^2 + x_2^2 + ... + x_n^2$ are the number of coordinates you have.

In this case we have 3. So the length or $\displaystyle AC = \sqrt{x^2 + y^2 + z^2} $

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.