Two points $A$ and $B$ in $\mathbb{R}^3$ with origin $O$ are given in terms of a Cartesian coordinate system by $A = (1, 2, 3)$ and $B = (4, 5, −1)$.
How do you find the point $C$, such that $OACB$ are the vertices of a parallelogram (with $A$ and $B$ diagonally opposite each other).
I have worked out $|OA| = \sqrt{14}$, $|OB| = \sqrt{42}$, $|AB| = \sqrt{34}$.