Answer
$$
M_O=\{-25 \mathbf{i}+6200 \mathbf{j}-900 \mathbf{k}\} \mathrm{lb} \cdot \mathrm{ft}
$$
Work Step by Step
The coordinates of point $B$ are $B(8,0.25,1.5) \mathrm{ft}$.
$$
\mathbf{r}_{O B}=\{8 \mathbf{i}+0.25 \mathbf{j}+1.5 \mathbf{k}\} \mathrm{ft}
$$
Moments of $\boldsymbol{F}$ About Point $\boldsymbol{O}$.
$$
\begin{aligned}
M_O & =\mathbf{r}_{O B} \times \mathbf{F} \\
& =\left|\begin{array}{ccc}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
8 & 0.25 & 1.5 \\
400 & -100 & -700
\end{array}\right| \\
& =\{-25 \mathbf{i}+6200 \mathbf{j}-900 \mathbf{k}\} \mathrm{lb} \cdot \mathrm{ft}
\end{aligned}
$$
