The direction vector $\vec{d}$ of a line passing through two points $P(x_1, y_1, z_1)$ and $Q(x_2, y_2, z_2)$ is given by $\vec{d} = Q - P = (x_2 - x_1, y_2 - y_1, z_2 - z_1)$. So, for the points $P(1,2,-1)$ and $Q(-1,0,1)$, we have $\vec{d} = Q - P = (-1 - 1, 0 -
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