Answer
$$
\mathbf{M}_A=\{-110 \mathbf{i}+70 \mathbf{j}-20 \mathbf{k}\} \mathrm{N} \cdot \mathrm{m}
$$
Work Step by Step
The coordinates of point $C$ are $C(0.5,0.7,-0.3) \mathrm{m}$.
$$
\mathbf{r}_{A C}=\{0.5 \mathbf{i}+0.7 \mathbf{j}-0.3 \mathbf{k}\} \mathrm{m}
$$
Moment of Force $\boldsymbol{F}$ About Point $\boldsymbol{A}$.
$$
\begin{aligned}
\mathbf{M}_A & =\mathbf{r}_{A C} \times \mathbf{F} \\
& =\left|\begin{array}{ccc}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
0.5 & 0.7 & -0.3 \\
600 & 800 & -500
\end{array}\right| \\
& =\{-110 \mathbf{i}+70 \mathbf{j}-20 \mathbf{k}\} \mathbf{N} \cdot \mathrm{m}
\end{aligned}
$$
