Answer
$$
\mathbf{M}_o=\{-40 \mathbf{i}-44 \mathbf{j}-8 \mathbf{k}\} \mathrm{kN} \cdot \mathrm{m}
$$
Work Step by Step
Position Vector. The coordinates of point $A$ are $(1,-2,6) \mathrm{m}$. Thus,
$$
\mathbf{r}_{O A}=\{\mathbf{i}-2 \mathbf{j}+6 \mathbf{k}\} \mathrm{m}
$$
The moment of $\boldsymbol{F}$ About Point $\boldsymbol{O}$.
$$
\begin{aligned}
\mathbf{M}_O & =\mathbf{r}_{O A} \times \mathbf{F} \\
& =\left|\begin{array}{ccc}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
1 & -2 & 6 \\
-6 & 4 & 8
\end{array}\right| \\
& =\{-40 \mathbf{i}-44 \mathbf{j}-8 \mathbf{k}\} \mathrm{kN} \cdot \mathrm{m}
\end{aligned}
$$
